# Class GregorianCalendar

java.lang.Object
java.util.Calendar
java.util.GregorianCalendar

All Implemented Interfaces:
  `Serializable, Cloneable, Comparable<Calendar>`

public class GregorianCalendar extends Calendar
`GregorianCalendar` is a concrete subclass of `Calendar` and provides the standard calendar system used by most of the world.

`GregorianCalendar` is a hybrid calendar that supports both the Julian and Gregorian calendar systems with the support of a single discontinuity, which corresponds by default to the Gregorian date when the Gregorian calendar was instituted (October 15, 1582 in some countries, later in others). The cutover date may be changed by the caller by calling `setGregorianChange()`.

Historically, in those countries which adopted the Gregorian calendar first, October 4, 1582 (Julian) was thus followed by October 15, 1582 (Gregorian). This calendar models this correctly. Before the Gregorian cutover, `GregorianCalendar` implements the Julian calendar. The only difference between the Gregorian and the Julian calendar is the leap year rule. The Julian calendar specifies leap years every four years, whereas the Gregorian calendar omits century years which are not divisible by 400.

`GregorianCalendar` implements proleptic Gregorian and Julian calendars. That is, dates are computed by extrapolating the current rules indefinitely far backward and forward in time. As a result, `GregorianCalendar` may be used for all years to generate meaningful and consistent results. However, dates obtained using `GregorianCalendar` are historically accurate only from March 1, 4 AD onward, when modern Julian calendar rules were adopted. Before this date, leap year rules were applied irregularly, and before 45 BC the Julian calendar did not even exist.

Prior to the institution of the Gregorian calendar, New Year's Day was March 25. To avoid confusion, this calendar always uses January 1. A manual adjustment may be made if desired for dates that are prior to the Gregorian changeover and which fall between January 1 and March 24.

## Week Of Year and Week Year

Values calculated for the `WEEK_OF_YEAR` field range from 1 to 53. The first week of a calendar year is the earliest seven day period starting on `getFirstDayOfWeek()` that contains at least `getMinimalDaysInFirstWeek()` days from that year. It thus depends on the values of `getMinimalDaysInFirstWeek()`, `getFirstDayOfWeek()`, and the day of the week of January 1. Weeks between week 1 of one year and week 1 of the following year (exclusive) are numbered sequentially from 2 to 52 or 53 (except for year(s) involved in the Julian-Gregorian transition).

The `getFirstDayOfWeek()` and `getMinimalDaysInFirstWeek()` values are initialized using locale-dependent resources when constructing a `GregorianCalendar`. The week determination is compatible with the ISO 8601 standard when `getFirstDayOfWeek()` is `MONDAY` and `getMinimalDaysInFirstWeek()` is 4, which values are used in locales where the standard is preferred. These values can explicitly be set by calling `setFirstDayOfWeek()` and `setMinimalDaysInFirstWeek()`.

A week year is in sync with a `WEEK_OF_YEAR` cycle. All weeks between the first and last weeks (inclusive) have the same week year value. Therefore, the first and last days of a week year may have different calendar year values.

For example, January 1, 1998 is a Thursday. If `getFirstDayOfWeek()` is `MONDAY` and `getMinimalDaysInFirstWeek()` is 4 (ISO 8601 standard compatible setting), then week 1 of 1998 starts on December 29, 1997, and ends on January 4, 1998. The week year is 1998 for the last three days of calendar year 1997. If, however, `getFirstDayOfWeek()` is `SUNDAY`, then week 1 of 1998 starts on January 4, 1998, and ends on January 10, 1998; the first three days of 1998 then are part of week 53 of 1997 and their week year is 1997.

### Week Of Month

Values calculated for the `WEEK_OF_MONTH` field range from 0 to 6. Week 1 of a month (the days with `WEEK_OF_MONTH = 1`) is the earliest set of at least `getMinimalDaysInFirstWeek()` contiguous days in that month, ending on the day before `getFirstDayOfWeek()`. Unlike week 1 of a year, week 1 of a month may be shorter than 7 days, need not start on `getFirstDayOfWeek()`, and will not include days of the previous month. Days of a month before week 1 have a `WEEK_OF_MONTH` of 0.

For example, if `getFirstDayOfWeek()` is `SUNDAY` and `getMinimalDaysInFirstWeek()` is 4, then the first week of January 1998 is Sunday, January 4 through Saturday, January 10. These days have a `WEEK_OF_MONTH` of 1. Thursday, January 1 through Saturday, January 3 have a `WEEK_OF_MONTH` of 0. If `getMinimalDaysInFirstWeek()` is changed to 3, then January 1 through January 3 have a `WEEK_OF_MONTH` of 1.

### Default Fields Values

The `clear` method sets calendar field(s) undefined. `GregorianCalendar` uses the following default value for each calendar field if its value is undefined.

GregorianCalendar default field values
Field | Default Value
--- | ---
`ERA` | `AD`
`YEAR` | `1970`
`MONTH` | `JANUARY`
`DAY_OF_MONTH` | `1`
`DAY_OF_WEEK` | `the first day of week`
`WEEK_OF_MONTH` | `0`
`DAY_OF_WEEK_IN_MONTH` | `1`
`AM_PM` | `AM`
`HOUR, HOUR_OF_DAY, MINUTE, SECOND, MILLISECOND` | `0`

Default values are not applicable for the fields not listed above.

Example:

>     // get the supported ids for GMT-08:00 (Pacific Standard Time)
>     String[] ids = TimeZone.getAvailableIDs(-8 * 60 * 60 * 1000);
>     // if no ids were returned, something is wrong. get out.
>     if (ids.length == 0)
>         System.exit(0);

>      // begin output
>     System.out.println("Current Time");

>     // create a Pacific Standard Time time zone
>     SimpleTimeZone pdt = new SimpleTimeZone(-8 * 60 * 60 * 1000, ids[0]);

>     // set up rules for Daylight Saving Time
>     pdt.setStartRule(Calendar.APRIL, 1, Calendar.SUNDAY, 2 * 60 * 60 * 1000);
>     pdt.setEndRule(Calendar.OCTOBER, -1, Calendar.SUNDAY, 2 * 60 * 60 * 1000);

>     // create a GregorianCalendar with the Pacific Daylight time zone
>     // and the current date and time
>     Calendar calendar = new GregorianCalendar(pdt);
>     Date trialTime = new Date();
>     calendar.setTime(trialTime);

>     // print out a bunch of interesting things
>     System.out.println("ERA: " + calendar.get(Calendar.ERA));
>     System.out.println("YEAR: " + calendar.get(Calendar.YEAR));
>     System.out.println("MONTH: " + calendar.get(Calendar.MONTH));
>     System.out.println("WEEK_OF_YEAR: " + calendar.get(Calendar.WEEK_OF_YEAR));
>     System.out.println("WEEK_OF_MONTH: " + calendar.get(Calendar.WEEK_OF_MONTH));
>     System.out.println("DATE: " + calendar.get(Calendar.DATE));
>     System.out.println("DAY_OF_MONTH: " + calendar.get(Calendar.DAY_OF_MONTH));
>     System.out.println("DAY_OF_YEAR: " + calendar.get(Calendar.DAY_OF_YEAR));
>     System.out.println("DAY_OF_WEEK: " + calendar.get(Calendar.DAY_OF_WEEK));
>     System.out.println("DAY_OF_WEEK_IN_MONTH: "
>                        + calendar.get(Calendar.DAY_OF_WEEK_IN_MONTH));
>     System.out.println("AM_PM: " + calendar.get(Calendar.AM_PM));
>     System.out.println("HOUR: " + calendar.get(Calendar.HOUR));
>     System.out.println("HOUR_OF_DAY: " + calendar.get(Calendar.HOUR_OF_DAY));
>     System.out.println("MINUTE: " + calendar.get(Calendar.MINUTE));
>     System.out.println("SECOND: " + calendar.get(Calendar.SECOND));
>     System.out.println("MILLISECOND: " + calendar.get(Calendar.MILLISECOND));
>     System.out.println("ZONE_OFFSET: "
>                        + (calendar.get(Calendar.ZONE_OFFSET)/(60*60*1000)));
>     System.out.println("DST_OFFSET: "
>                        + (calendar.get(Calendar.DST_OFFSET)/(60*60*1000)));
>     System.out.println("Current Time, with hour reset to 3");
>     calendar.clear(Calendar.HOUR_OF_DAY); // so doesn't override
>     calendar.set(Calendar.HOUR, 3);
>     System.out.println("ERA: " + calendar.get(Calendar.ERA));
>     System.out.println("YEAR: " + calendar.get(Calendar.YEAR));
>     System.out.println("MONTH: " + calendar.get(Calendar.MONTH));
>     System.out.println("WEEK_OF_YEAR: " + calendar.get(Calendar.WEEK_OF_YEAR));
>     System.out.println("WEEK_OF_MONTH: " + calendar.get(Calendar.WEEK_OF_MONTH));
>     System.out.println("DATE: " + calendar.get(Calendar.DATE));
>     System.out.println("DAY_OF_MONTH: " + calendar.get(Calendar.DAY_OF_MONTH));
>     System.out.println("DAY_OF_YEAR: " + calendar.get(Calendar.DAY_OF_YEAR));
>     System.out.println("DAY_OF_WEEK: " + calendar.get(Calendar.DAY_OF_WEEK));
>     System.out.println("DAY_OF_WEEK_IN_MONTH: "
>                        + calendar.get(Calendar.DAY_OF_WEEK_IN_MONTH));
>     System.out.println("AM_PM: " + calendar.get(Calendar.AM_PM));
>     System.out.println("HOUR: " + calendar.get(Calendar.HOUR));
>     System.out.println("HOUR_OF_DAY: " + calendar.get(Calendar.HOUR_OF_DAY));
>     System.out.println("MINUTE: " + calendar.get(Calendar.MINUTE));
>     System.out.println("SECOND: " + calendar.get(Calendar.SECOND));
>     System.out.println("MILLISECOND: " + calendar.get(Calendar.MILLISECOND));
>     System.out.println("ZONE_OFFSET: "
>            + (calendar.get(Calendar.ZONE_OFFSET)/(60*60*1000))); // in hours
>     System.out.println("DST_OFFSET: "
>            + (calendar.get(Calendar.DST_OFFSET)/(60*60*1000))); // in hours

Since:
  1.1
See Also:
  - `TimeZone`
  - Serialized Form

- ## Nested Class Summary

  ### Nested classes/interfaces declared in class Calendar

  `Calendar.Builder`
  Modifier and Type
  Class
  Description
  `static class`
  `Calendar.Builder`
  `Calendar.Builder` is used for creating a `Calendar` from various date-time parameters.

- ## Field Summary

  Fields
  Modifier and Type
  Field
  Description
  `static final int`
  `AD`
  Value of the `ERA` field indicating the common era (Anno Domini), also known as CE.
  `static final int`
  `BC`
  Value of the `ERA` field indicating the period before the common era (before Christ), also known as BCE.

  ### Fields declared in class Calendar

  `ALL_STYLES, AM, AM_PM, APRIL, areFieldsSet, AUGUST, DATE, DAY_OF_MONTH, DAY_OF_WEEK, DAY_OF_WEEK_IN_MONTH, DAY_OF_YEAR, DECEMBER, DST_OFFSET, ERA, FEBRUARY, FIELD_COUNT, fields, FRIDAY, HOUR, HOUR_OF_DAY, isSet, isTimeSet, JANUARY, JULY, JUNE, LONG, LONG_FORMAT, LONG_STANDALONE, MARCH, MAY, MILLISECOND, MINUTE, MONDAY, MONTH, NARROW_FORMAT, NARROW_STANDALONE, NOVEMBER, OCTOBER, PM, SATURDAY, SECOND, SEPTEMBER, SHORT, SHORT_FORMAT, SHORT_STANDALONE, SUNDAY, THURSDAY, time, TUESDAY, UNDECIMBER, WEDNESDAY, WEEK_OF_MONTH, WEEK_OF_YEAR, YEAR, ZONE_OFFSET`
  Modifier and Type
  Field
  Description
  `static final int`
  `ALL_STYLES`
  A style specifier for `getDisplayNames` indicating names in all styles, such as "January" and "Jan".
  `static final int`
  `AM`
  Value of the `Calendar.AM_PM` field indicating the period of the day from midnight to just before noon.
  `static final int`
  `AM_PM`
  Field number for `get` and `set` indicating whether the `HOUR` is before or after noon.
  `static final int`
  `APRIL`
  Value of the `Calendar.MONTH` field indicating the fourth month of the year in the Gregorian and Julian calendars.
  `protected boolean`
  `areFieldsSet`
  True if `fields[]` are in sync with the currently set time.
  `static final int`
  `AUGUST`
  Value of the `Calendar.MONTH` field indicating the eighth month of the year in the Gregorian and Julian calendars.
  `static final int`
  `DATE`
  Field number for `get` and `set` indicating the day of the month.
  `static final int`
  `DAY_OF_MONTH`
  Field number for `get` and `set` indicating the day of the month.
  `static final int`
  `DAY_OF_WEEK`
  Field number for `get` and `set` indicating the day of the week.
  `static final int`
  `DAY_OF_WEEK_IN_MONTH`
  Field number for `get` and `set` indicating the ordinal number of the day of the week within the current month.
  `static final int`
  `DAY_OF_YEAR`
  Field number for `get` and `set` indicating the day number within the current year.
  `static final int`
  `DECEMBER`
  Value of the `Calendar.MONTH` field indicating the twelfth month of the year in the Gregorian and Julian calendars.
  `static final int`
  `DST_OFFSET`
  Field number for `get` and `set` indicating the daylight saving offset in milliseconds.
  `static final int`
  `ERA`
  Field number for `get` and `set` indicating the era, e.g., AD or BC in the Julian calendar.
  `static final int`
  `FEBRUARY`
  Value of the `Calendar.MONTH` field indicating the second month of the year in the Gregorian and Julian calendars.
  `static final int`
  `FIELD_COUNT`
  The number of distinct fields recognized by `get` and `set`.
  `protected int[]`
  `fields`
  The calendar field values for the currently set time for this calendar.
  `static final int`
  `FRIDAY`
  Value of the `Calendar.DAY_OF_WEEK` field indicating Friday.
  `static final int`
  `HOUR`
  Field number for `get` and `set` indicating the hour of the morning or afternoon.
  `static final int`
  `HOUR_OF_DAY`
  Field number for `get` and `set` indicating the hour of the day.
  `protected boolean[]`
  `isSet`
  The flags which tell if a specified calendar field for the calendar is set.
  `protected boolean`
  `isTimeSet`
  True if then the value of `time` is valid.
  `static final int`
  `JANUARY`
  Value of the `Calendar.MONTH` field indicating the first month of the year in the Gregorian and Julian calendars.
  `static final int`
  `JULY`
  Value of the `Calendar.MONTH` field indicating the seventh month of the year in the Gregorian and Julian calendars.
  `static final int`
  `JUNE`
  Value of the `Calendar.MONTH` field indicating the sixth month of the year in the Gregorian and Julian calendars.
  `static final int`
  `LONG`
  A style specifier for `getDisplayName` and `getDisplayNames` equivalent to `Calendar.LONG_FORMAT`.
  `static final int`
  `LONG_FORMAT`
  A style specifier for `getDisplayName` and `getDisplayNames` indicating a long name used for format.
  `static final int`
  `LONG_STANDALONE`
  A style specifier for `getDisplayName` and `getDisplayNames` indicating a long name used independently, such as a month name as calendar headers.
  `static final int`
  `MARCH`
  Value of the `Calendar.MONTH` field indicating the third month of the year in the Gregorian and Julian calendars.
  `static final int`
  `MAY`
  Value of the `Calendar.MONTH` field indicating the fifth month of the year in the Gregorian and Julian calendars.
  `static final int`
  `MILLISECOND`
  Field number for `get` and `set` indicating the millisecond within the second.
  `static final int`
  `MINUTE`
  Field number for `get` and `set` indicating the minute within the hour.
  `static final int`
  `MONDAY`
  Value of the `Calendar.DAY_OF_WEEK` field indicating Monday.
  `static final int`
  `MONTH`
  Field number for `get` and `set` indicating the month.
  `static final int`
  `NARROW_FORMAT`
  A style specifier for `getDisplayName` and `getDisplayNames` indicating a narrow name used for format.
  `static final int`
  `NARROW_STANDALONE`
  A style specifier for `getDisplayName` and `getDisplayNames` indicating a narrow name independently.
  `static final int`
  `NOVEMBER`
  Value of the `Calendar.MONTH` field indicating the eleventh month of the year in the Gregorian and Julian calendars.
  `static final int`
  `OCTOBER`
  Value of the `Calendar.MONTH` field indicating the tenth month of the year in the Gregorian and Julian calendars.
  `static final int`
  `PM`
  Value of the `Calendar.AM_PM` field indicating the period of the day from noon to just before midnight.
  `static final int`
  `SATURDAY`
  Value of the `Calendar.DAY_OF_WEEK` field indicating Saturday.
  `static final int`
  `SECOND`
  Field number for `get` and `set` indicating the second within the minute.
  `static final int`
  `SEPTEMBER`
  Value of the `Calendar.MONTH` field indicating the ninth month of the year in the Gregorian and Julian calendars.
  `static final int`
  `SHORT`
  A style specifier for `getDisplayName` and `getDisplayNames` equivalent to `Calendar.SHORT_FORMAT`.
  `static final int`
  `SHORT_FORMAT`
  A style specifier for `getDisplayName` and `getDisplayNames` indicating a short name used for format.
  `static final int`
  `SHORT_STANDALONE`
  A style specifier for `getDisplayName` and `getDisplayNames` indicating a short name used independently, such as a month abbreviation as calendar headers.
  `static final int`
  `SUNDAY`
  Value of the `Calendar.DAY_OF_WEEK` field indicating Sunday.
  `static final int`
  `THURSDAY`
  Value of the `Calendar.DAY_OF_WEEK` field indicating Thursday.
  `protected long`
  `time`
  The currently set time for this calendar, expressed in milliseconds after January 1, 1970, 0:00:00 GMT.
  `static final int`
  `TUESDAY`
  Value of the `Calendar.DAY_OF_WEEK` field indicating Tuesday.
  `static final int`
  `UNDECIMBER`
  Value of the `Calendar.MONTH` field indicating the thirteenth month of the year.
  `static final int`
  `WEDNESDAY`
  Value of the `Calendar.DAY_OF_WEEK` field indicating Wednesday.
  `static final int`
  `WEEK_OF_MONTH`
  Field number for `get` and `set` indicating the week number within the current month.
  `static final int`
  `WEEK_OF_YEAR`
  Field number for `get` and `set` indicating the week number within the current year.
  `static final int`
  `YEAR`
  Field number for `get` and `set` indicating the year.
  `static final int`
  `ZONE_OFFSET`
  Field number for `get` and `set` indicating the raw offset from GMT in milliseconds.

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `GregorianCalendar()`
  Constructs a default `GregorianCalendar` using the current time in the default time zone with the default `FORMAT` locale.
  `GregorianCalendar(int year, int month, int dayOfMonth)`
  Constructs a `GregorianCalendar` with the given date set in the default time zone with the default locale.
  `GregorianCalendar(int year, int month, int dayOfMonth, int hourOfDay, int minute)`
  Constructs a `GregorianCalendar` with the given date and time set for the default time zone with the default locale.
  `GregorianCalendar(int year, int month, int dayOfMonth, int hourOfDay, int minute, int second)`
  Constructs a GregorianCalendar with the given date and time set for the default time zone with the default locale.
  `GregorianCalendar(Locale aLocale)`
  Constructs a `GregorianCalendar` based on the current time in the default time zone with the given locale.
  `GregorianCalendar(TimeZone zone)`
  Constructs a `GregorianCalendar` based on the current time in the given time zone with the default `FORMAT` locale.
  `GregorianCalendar(TimeZone zone, Locale aLocale)`
  Constructs a `GregorianCalendar` based on the current time in the given time zone with the given locale.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `add(int field, int amount)`
  Adds the specified (signed) amount of time to the given calendar field, based on the calendar's rules.
  `protected void`
  `computeFields()`
  Converts the time value (millisecond offset from the Epoch) to calendar field values.
  `protected void`
  `computeTime()`
  Converts calendar field values to the time value (millisecond offset from the Epoch).
  `boolean`
  `equals(Object obj)`
  Compares this `GregorianCalendar` to the specified `Object`.
  `static GregorianCalendar`
  `from(ZonedDateTime zdt)`
  Obtains an instance of `GregorianCalendar` with the default locale from a `ZonedDateTime` object.
  `int`
  `getActualMaximum(int field)`
  Returns the maximum value that this calendar field could have, taking into consideration the given time value and the current values of the `getFirstDayOfWeek`, `getMinimalDaysInFirstWeek`, `getGregorianChange` and `getTimeZone` methods.
  `int`
  `getActualMinimum(int field)`
  Returns the minimum value that this calendar field could have, taking into consideration the given time value and the current values of the `getFirstDayOfWeek`, `getMinimalDaysInFirstWeek`, `getGregorianChange` and `getTimeZone` methods.
  `String`
  `getCalendarType()`
  Returns `"gregory"` as the calendar type.
  `int`
  `getGreatestMinimum(int field)`
  Returns the highest minimum value for the given calendar field of this `GregorianCalendar` instance.
  `final Date`
  `getGregorianChange()`
  Gets the Gregorian Calendar change date.
  `int`
  `getLeastMaximum(int field)`
  Returns the lowest maximum value for the given calendar field of this `GregorianCalendar` instance.
  `int`
  `getMaximum(int field)`
  Returns the maximum value for the given calendar field of this `GregorianCalendar` instance.
  `int`
  `getMinimum(int field)`
  Returns the minimum value for the given calendar field of this `GregorianCalendar` instance.
  `int`
  `getWeeksInWeekYear()`
  Returns the number of weeks in the week year represented by this `GregorianCalendar`.
  `int`
  `getWeekYear()`
  Returns the week year represented by this `GregorianCalendar`.
  `int`
  `hashCode()`
  Generates the hash code for this `GregorianCalendar` object.
  `boolean`
  `isLeapYear(int year)`
  Determines if the given year is a leap year.
  `final boolean`
  `isWeekDateSupported()`
  Returns `true` indicating this `GregorianCalendar` supports week dates.
  `void`
  `roll(int field, boolean up)`
  Adds or subtracts (up/down) a single unit of time on the given time field without changing larger fields.
  `void`
  `roll(int field, int amount)`
  Adds a signed amount to the specified calendar field without changing larger fields.
  `void`
  `setGregorianChange(Date date)`
  Sets the `GregorianCalendar` change date.
  `void`
  `setWeekDate(int weekYear, int weekOfYear, int dayOfWeek)`
  Sets this `GregorianCalendar` to the date given by the date specifiers - `weekYear`, `weekOfYear`, and `dayOfWeek`.
  `ZonedDateTime`
  `toZonedDateTime()`
  Converts this object to a `ZonedDateTime` that represents the same point on the time-line as this `GregorianCalendar`.

  ### Methods declared in class Calendar

  `after, before, clear, clear, clone, compareTo, complete, get, getAvailableCalendarTypes, getAvailableLocales, getDisplayName, getDisplayNames, getFirstDayOfWeek, getInstance, getInstance, getInstance, getInstance, getMinimalDaysInFirstWeek, getTime, getTimeInMillis, getTimeZone, internalGet, isLenient, isSet, set, set, set, set, setFirstDayOfWeek, setLenient, setMinimalDaysInFirstWeek, setTime, setTimeInMillis, setTimeZone, toInstant, toString`
  Modifier and Type
  Method
  Description
  `boolean`
  `after(Object when)`
  Returns whether this `Calendar` represents a time after the time represented by the specified `Object`.
  `boolean`
  `before(Object when)`
  Returns whether this `Calendar` represents a time before the time represented by the specified `Object`.
  `final void`
  `clear()`
  Sets all the calendar field values and the time value (millisecond offset from the Epoch) of this `Calendar` undefined.
  `final void`
  `clear(int field)`
  Sets the given calendar field value and the time value (millisecond offset from the Epoch) of this `Calendar` undefined.
  `Object`
  `clone()`
  Creates and returns a copy of this object.
  `int`
  `compareTo(Calendar anotherCalendar)`
  Compares the time values (millisecond offsets from the Epoch) represented by two `Calendar` objects.
  `protected void`
  `complete()`
  Fills in any unset fields in the calendar fields.
  `int`
  `get(int field)`
  Returns the value of the given calendar field.
  `static Set<String>`
  `getAvailableCalendarTypes()`
  Returns an unmodifiable `Set` containing all calendar types supported by `Calendar` in the runtime environment.
  `static Locale[]`
  `getAvailableLocales()`
  Returns an array of all locales for which the `getInstance` methods of this class can return localized instances.
  `String`
  `getDisplayName(int field, int style, Locale locale)`
  Returns the string representation of the calendar `field` value in the given `style` and `locale`.
  `Map<String,Integer>`
  `getDisplayNames(int field, int style, Locale locale)`
  Returns a `Map` containing all names of the calendar `field` in the given `style` and `locale` and their corresponding field values.
  `int`
  `getFirstDayOfWeek()`
  Gets what the first day of the week is; e.g., `SUNDAY` in the U.S., `MONDAY` in France.
  `static Calendar`
  `getInstance()`
  Gets a calendar using the default time zone and locale.
  `static Calendar`
  `getInstance(Locale aLocale)`
  Gets a calendar using the default time zone and specified locale.
  `static Calendar`
  `getInstance(TimeZone zone)`
  Gets a calendar using the specified time zone and default locale.
  `static Calendar`
  `getInstance(TimeZone zone, Locale aLocale)`
  Gets a calendar with the specified time zone and locale.
  `int`
  `getMinimalDaysInFirstWeek()`
  Gets what the minimal days required in the first week of the year are; e.g., if the first week is defined as one that contains the first day of the first month of a year, this method returns 1.
  `final Date`
  `getTime()`
  Returns a `Date` object representing this `Calendar`'s time value (millisecond offset from the Epoch").
  `long`
  `getTimeInMillis()`
  Returns this Calendar's time value in milliseconds.
  `TimeZone`
  `getTimeZone()`
  Gets the time zone.
  `protected final int`
  `internalGet(int field)`
  Returns the value of the given calendar field.
  `boolean`
  `isLenient()`
  Tells whether date/time interpretation is to be lenient.
  `final boolean`
  `isSet(int field)`
  Determines if the given calendar field has a value set, including cases that the value has been set by internal fields calculations triggered by a `get` method call.
  `void`
  `set(int field, int value)`
  Sets the given calendar field to the given value.
  `final void`
  `set(int year, int month, int date)`
  Sets the values for the calendar fields `YEAR`, `MONTH`, and `DAY_OF_MONTH`.
  `final void`
  `set(int year, int month, int date, int hourOfDay, int minute)`
  Sets the values for the calendar fields `YEAR`, `MONTH`, `DAY_OF_MONTH`, `HOUR_OF_DAY`, and `MINUTE`.
  `final void`
  `set(int year, int month, int date, int hourOfDay, int minute, int second)`
  Sets the values for the fields `YEAR`, `MONTH`, `DAY_OF_MONTH`, `HOUR_OF_DAY`, `MINUTE`, and `SECOND`.
  `void`
  `setFirstDayOfWeek(int value)`
  Sets what the first day of the week is; e.g., `SUNDAY` in the U.S., `MONDAY` in France.
  `void`
  `setLenient(boolean lenient)`
  Specifies whether or not date/time interpretation is to be lenient.
  `void`
  `setMinimalDaysInFirstWeek(int value)`
  Sets what the minimal days required in the first week of the year are; For example, if the first week is defined as one that contains the first day of the first month of a year, call this method with value 1.
  `final void`
  `setTime(Date date)`
  Sets this Calendar's time with the given `Date`.
  `void`
  `setTimeInMillis(long millis)`
  Sets this Calendar's current time from the given long value.
  `void`
  `setTimeZone(TimeZone value)`
  Sets the time zone with the given time zone value.
  `final Instant`
  `toInstant()`
  Converts this object to an `Instant`.
  `String`
  `toString()`
  Return a string representation of this calendar.

  ### Methods declared in class Object

  `finalize, getClass, notify, notifyAll, wait, wait, wait`
  Modifier and Type
  Method
  Description
  `protected void`
  `finalize()`
  Deprecated, for removal: This API element is subject to removal in a future version.
  Finalization is deprecated and subject to removal in a future release.
  `final Class<?>`
  `getClass()`
  Returns the runtime class of this `Object`.
  `final void`
  `notify()`
  Wakes up a single thread that is waiting on this object's monitor.
  `final void`
  `notifyAll()`
  Wakes up all threads that are waiting on this object's monitor.
  `final void`
  `wait()`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted.
  `final void`
  `wait(long timeoutMillis)`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
  `final void`
  `wait(long timeoutMillis, int nanos)`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.

- ## Field Details

  - ### BC

    public static final int BC
    Value of the `ERA` field indicating the period before the common era (before Christ), also known as BCE. The sequence of years at the transition from `BC` to `AD` is ..., 2 BC, 1 BC, 1 AD, 2 AD,...
    See Also:
      - `Calendar.ERA`
      - Constant Field Values

  - ### AD

    public static final int AD
    Value of the `ERA` field indicating the common era (Anno Domini), also known as CE. The sequence of years at the transition from `BC` to `AD` is ..., 2 BC, 1 BC, 1 AD, 2 AD,...
    See Also:
      - `Calendar.ERA`
      - Constant Field Values

- ## Constructor Details

  - ### GregorianCalendar

    public GregorianCalendar()
    Constructs a default `GregorianCalendar` using the current time in the default time zone with the default `FORMAT` locale.

  - ### GregorianCalendar

    public GregorianCalendar(TimeZone zone)
    Constructs a `GregorianCalendar` based on the current time in the given time zone with the default `FORMAT` locale.
    Parameters:
      `zone` - the given time zone.
    Throws:
      `NullPointerException` - if `zone` is `null`

  - ### GregorianCalendar

    public GregorianCalendar(Locale aLocale)
    Constructs a `GregorianCalendar` based on the current time in the default time zone with the given locale.
    Parameters:
      `aLocale` - the given locale.
    Throws:
      `NullPointerException` - if `aLocale` is `null`

  - ### GregorianCalendar

    public GregorianCalendar(TimeZone zone, Locale aLocale)
    Constructs a `GregorianCalendar` based on the current time in the given time zone with the given locale.
    Parameters:
      `zone` - the given time zone.
      `aLocale` - the given locale.
    Throws:
      `NullPointerException` - if `zone` or `aLocale` is `null`

  - ### GregorianCalendar

    public GregorianCalendar(int year, int month, int dayOfMonth)
    Constructs a `GregorianCalendar` with the given date set in the default time zone with the default locale.
    Parameters:
      `year` - the value used to set the `YEAR` calendar field in the calendar.
      `month` - the value used to set the `MONTH` calendar field in the calendar. Month value is 0-based. e.g., 0 for January.
      `dayOfMonth` - the value used to set the `DAY_OF_MONTH` calendar field in the calendar.

  - ### GregorianCalendar

    public GregorianCalendar(int year, int month, int dayOfMonth, int hourOfDay, int minute)
    Constructs a `GregorianCalendar` with the given date and time set for the default time zone with the default locale.
    Parameters:
      `year` - the value used to set the `YEAR` calendar field in the calendar.
      `month` - the value used to set the `MONTH` calendar field in the calendar. Month value is 0-based. e.g., 0 for January.
      `dayOfMonth` - the value used to set the `DAY_OF_MONTH` calendar field in the calendar.
      `hourOfDay` - the value used to set the `HOUR_OF_DAY` calendar field in the calendar.
      `minute` - the value used to set the `MINUTE` calendar field in the calendar.

  - ### GregorianCalendar

    public GregorianCalendar(int year, int month, int dayOfMonth, int hourOfDay, int minute, int second)
    Constructs a GregorianCalendar with the given date and time set for the default time zone with the default locale.
    Parameters:
      `year` - the value used to set the `YEAR` calendar field in the calendar.
      `month` - the value used to set the `MONTH` calendar field in the calendar. Month value is 0-based. e.g., 0 for January.
      `dayOfMonth` - the value used to set the `DAY_OF_MONTH` calendar field in the calendar.
      `hourOfDay` - the value used to set the `HOUR_OF_DAY` calendar field in the calendar.
      `minute` - the value used to set the `MINUTE` calendar field in the calendar.
      `second` - the value used to set the `SECOND` calendar field in the calendar.

- ## Method Details

  - ### setGregorianChange

    public void setGregorianChange(Date date)
    Sets the `GregorianCalendar` change date. This is the point when the switch from Julian dates to Gregorian dates occurred. Default is October 15, 1582 (Gregorian). Previous to this, dates will be in the Julian calendar.

    To obtain a pure Julian calendar, set the change date to `Date(Long.MAX_VALUE)`. To obtain a pure Gregorian calendar, set the change date to `Date(Long.MIN_VALUE)`.

    Parameters:
      `date` - the given Gregorian cutover date.

  - ### getGregorianChange

    public final Date getGregorianChange()
    Gets the Gregorian Calendar change date. This is the point when the switch from Julian dates to Gregorian dates occurred. Default is October 15, 1582 (Gregorian). Previous to this, dates will be in the Julian calendar.
    Returns:
      the Gregorian cutover date for this `GregorianCalendar` object.

  - ### isLeapYear

    public boolean isLeapYear(int year)
    Determines if the given year is a leap year. Returns `true` if the given year is a leap year. To specify BC year numbers, `1 - year number` must be given. For example, year BC 4 is specified as -3.
    Parameters:
      `year` - the given year.
    Returns:
      `true` if the given year is a leap year; `false` otherwise.

  - ### getCalendarType

    public String getCalendarType()
    Returns `"gregory"` as the calendar type.
    Overrides:
      `getCalendarType` in class `Calendar`
    Returns:
      `"gregory"`
    Since:
      1.8
    See Also:
      - Locale extensions
      - `Locale.Builder.setLocale(Locale)`
      - `Locale.Builder.setUnicodeLocaleKeyword(String, String)`

  - ### equals

    public boolean equals(Object obj)
    Compares this `GregorianCalendar` to the specified `Object`. The result is `true` if and only if the argument is a `GregorianCalendar` object that represents the same time value (millisecond offset from the Epoch) under the same `Calendar` parameters and Gregorian change date as this object.
    Overrides:
      `equals` in class `Calendar`
    Parameters:
      `obj` - the object to compare with.
    Returns:
      `true` if this object is equal to `obj`; `false` otherwise.
    See Also:
      - `Calendar.compareTo(Calendar)`

  - ### hashCode

    public int hashCode()
    Generates the hash code for this `GregorianCalendar` object.
    Overrides:
      `hashCode` in class `Calendar`
    Returns:
      a hash code value for this object.
    See Also:
      - `Object.equals(java.lang.Object)`
      - `System.identityHashCode(Object)`

  - ### add

    public void add(int field, int amount)
    Adds the specified (signed) amount of time to the given calendar field, based on the calendar's rules.

    Add rule 1. The value of `field` after the call minus the value of `field` before the call is `amount`, modulo any overflow that has occurred in `field`. Overflow occurs when a field value exceeds its range and, as a result, the next larger field is incremented or decremented and the field value is adjusted back into its range.

    Add rule 2. If a smaller field is expected to be invariant, but it is impossible for it to be equal to its prior value because of changes in its minimum or maximum after `field` is changed, then its value is adjusted to be as close as possible to its expected value. A smaller field represents a smaller unit of time. `HOUR` is a smaller field than `DAY_OF_MONTH`. No adjustment is made to smaller fields that are not expected to be invariant. The calendar system determines what fields are expected to be invariant.

    Specified by:
      `add` in class `Calendar`
    Parameters:
      `field` - the calendar field.
      `amount` - the amount of date or time to be added to the field.
    Throws:
      `IllegalArgumentException` - if `field` is `ZONE_OFFSET`, `DST_OFFSET`, or unknown, or if any calendar fields have out-of-range values in non-lenient mode.
    See Also:
      - `Calendar.roll(int,int)`
      - `Calendar.set(int,int)`

  - ### roll

    public void roll(int field, boolean up)
    Adds or subtracts (up/down) a single unit of time on the given time field without changing larger fields.

    Example: Consider a `GregorianCalendar` originally set to December 31, 1999. Calling `roll(Calendar.MONTH, true)` sets the calendar to January 31, 1999. The `YEAR` field is unchanged because it is a larger field than `MONTH`.

    Specified by:
      `roll` in class `Calendar`
    Parameters:
      `field` - the time field.
      `up` - indicates if the value of the specified calendar field is to be rolled up or rolled down. Use `true` if rolling up, `false` otherwise.
    Throws:
      `IllegalArgumentException` - if `field` is `ZONE_OFFSET`, `DST_OFFSET`, or unknown, or if any calendar fields have out-of-range values in non-lenient mode.
    See Also:
      - `add(int,int)`
      - `Calendar.set(int,int)`

  - ### roll

    public void roll(int field, int amount)
    Adds a signed amount to the specified calendar field without changing larger fields. A negative roll amount means to subtract from field without changing larger fields. If the specified amount is 0, this method performs nothing.

    This method calls `Calendar.complete()` before adding the amount so that all the calendar fields are normalized. If there is any calendar field having an out-of-range value in non-lenient mode, then an `IllegalArgumentException` is thrown.

    Example: Consider a `GregorianCalendar` originally set to August 31, 1999. Calling `roll(Calendar.MONTH, 8)` sets the calendar to April 30, 1999. Using a `GregorianCalendar`, the `DAY_OF_MONTH` field cannot be 31 in the month April. `DAY_OF_MONTH` is set to the closest possible value, 30. The `YEAR` field maintains the value of 1999 because it is a larger field than `MONTH`.

    Example: Consider a `GregorianCalendar` originally set to Sunday June 6, 1999. Calling `roll(Calendar.WEEK_OF_MONTH, -1)` sets the calendar to Tuesday June 1, 1999, whereas calling `add(Calendar.WEEK_OF_MONTH, -1)` sets the calendar to Sunday May 30, 1999. This is because the roll rule imposes an additional constraint: The `MONTH` must not change when the `WEEK_OF_MONTH` is rolled. Taken together with add rule 1, the resultant date must be between Tuesday June 1 and Saturday June 5. According to add rule 2, the `DAY_OF_WEEK`, an invariant when changing the `WEEK_OF_MONTH`, is set to Tuesday, the closest possible value to Sunday (where Sunday is the first day of the week).

    Overrides:
      `roll` in class `Calendar`
    Parameters:
      `field` - the calendar field.
      `amount` - the signed amount to add to `field`.
    Throws:
      `IllegalArgumentException` - if `field` is `ZONE_OFFSET`, `DST_OFFSET`, or unknown, or if any calendar fields have out-of-range values in non-lenient mode.
    Since:
      1.2
    See Also:
      - `roll(int,boolean)`
      - `add(int,int)`
      - `Calendar.set(int,int)`

  - ### getMinimum

    public int getMinimum(int field)
    Returns the minimum value for the given calendar field of this `GregorianCalendar` instance. The minimum value is defined as the smallest value returned by the `get` method for any possible time value, taking into consideration the current values of the `getFirstDayOfWeek`, `getMinimalDaysInFirstWeek`, `getGregorianChange` and `getTimeZone` methods.
    Specified by:
      `getMinimum` in class `Calendar`
    Parameters:
      `field` - the calendar field.
    Returns:
      the minimum value for the given calendar field.
    See Also:
      - `getMaximum(int)`
      - `getGreatestMinimum(int)`
      - `getLeastMaximum(int)`
      - `getActualMinimum(int)`
      - `getActualMaximum(int)`

  - ### getMaximum

    public int getMaximum(int field)
    Returns the maximum value for the given calendar field of this `GregorianCalendar` instance. The maximum value is defined as the largest value returned by the `get` method for any possible time value, taking into consideration the current values of the `getFirstDayOfWeek`, `getMinimalDaysInFirstWeek`, `getGregorianChange` and `getTimeZone` methods.
    Specified by:
      `getMaximum` in class `Calendar`
    Parameters:
      `field` - the calendar field.
    Returns:
      the maximum value for the given calendar field.
    See Also:
      - `getMinimum(int)`
      - `getGreatestMinimum(int)`
      - `getLeastMaximum(int)`
      - `getActualMinimum(int)`
      - `getActualMaximum(int)`

  - ### getGreatestMinimum

    public int getGreatestMinimum(int field)
    Returns the highest minimum value for the given calendar field of this `GregorianCalendar` instance. The highest minimum value is defined as the largest value returned by `getActualMinimum(int)` for any possible time value, taking into consideration the current values of the `getFirstDayOfWeek`, `getMinimalDaysInFirstWeek`, `getGregorianChange` and `getTimeZone` methods.
    Specified by:
      `getGreatestMinimum` in class `Calendar`
    Parameters:
      `field` - the calendar field.
    Returns:
      the highest minimum value for the given calendar field.
    See Also:
      - `getMinimum(int)`
      - `getMaximum(int)`
      - `getLeastMaximum(int)`
      - `getActualMinimum(int)`
      - `getActualMaximum(int)`

  - ### getLeastMaximum

    public int getLeastMaximum(int field)
    Returns the lowest maximum value for the given calendar field of this `GregorianCalendar` instance. The lowest maximum value is defined as the smallest value returned by `getActualMaximum(int)` for any possible time value, taking into consideration the current values of the `getFirstDayOfWeek`, `getMinimalDaysInFirstWeek`, `getGregorianChange` and `getTimeZone` methods.
    Specified by:
      `getLeastMaximum` in class `Calendar`
    Parameters:
      `field` - the calendar field
    Returns:
      the lowest maximum value for the given calendar field.
    See Also:
      - `getMinimum(int)`
      - `getMaximum(int)`
      - `getGreatestMinimum(int)`
      - `getActualMinimum(int)`
      - `getActualMaximum(int)`

  - ### getActualMinimum

    public int getActualMinimum(int field)
    Returns the minimum value that this calendar field could have, taking into consideration the given time value and the current values of the `getFirstDayOfWeek`, `getMinimalDaysInFirstWeek`, `getGregorianChange` and `getTimeZone` methods.

    For example, if the Gregorian change date is January 10, 1970 and the date of this `GregorianCalendar` is January 20, 1970, the actual minimum value of the `DAY_OF_MONTH` field is 10 because the previous date of January 10, 1970 is December 27, 1996 (in the Julian calendar). Therefore, December 28, 1969 to January 9, 1970 don't exist.

    Overrides:
      `getActualMinimum` in class `Calendar`
    Parameters:
      `field` - the calendar field
    Returns:
      the minimum of the given field for the time value of this `GregorianCalendar`
    Since:
      1.2
    See Also:
      - `getMinimum(int)`
      - `getMaximum(int)`
      - `getGreatestMinimum(int)`
      - `getLeastMaximum(int)`
      - `getActualMaximum(int)`

  - ### getActualMaximum

    public int getActualMaximum(int field)
    Returns the maximum value that this calendar field could have, taking into consideration the given time value and the current values of the `getFirstDayOfWeek`, `getMinimalDaysInFirstWeek`, `getGregorianChange` and `getTimeZone` methods. For example, if the date of this instance is February 1, 2004, the actual maximum value of the `DAY_OF_MONTH` field is 29 because 2004 is a leap year, and if the date of this instance is February 1, 2005, it's 28.

    This method calculates the maximum value of `WEEK_OF_YEAR` based on the `YEAR` (calendar year) value, not the week year. Call `getWeeksInWeekYear()` to get the maximum value of `WEEK_OF_YEAR` in the week year of this `GregorianCalendar`.

    Overrides:
      `getActualMaximum` in class `Calendar`
    Parameters:
      `field` - the calendar field
    Returns:
      the maximum of the given field for the time value of this `GregorianCalendar`
    Since:
      1.2
    See Also:
      - `getMinimum(int)`
      - `getMaximum(int)`
      - `getGreatestMinimum(int)`
      - `getLeastMaximum(int)`
      - `getActualMinimum(int)`

  - ### isWeekDateSupported

    public final boolean isWeekDateSupported()
    Returns `true` indicating this `GregorianCalendar` supports week dates.
    Overrides:
      `isWeekDateSupported` in class `Calendar`
    Returns:
      `true` (always)
    Since:
      1.7
    See Also:
      - `getWeekYear()`
      - `setWeekDate(int,int,int)`
      - `getWeeksInWeekYear()`

  - ### getWeekYear

    public int getWeekYear()
    Returns the week year represented by this `GregorianCalendar`. The dates in the weeks between 1 and the maximum week number of the week year have the same week year value that may be one year before or after the `YEAR` (calendar year) value.

    This method calls `Calendar.complete()` before calculating the week year.

    Overrides:
      `getWeekYear` in class `Calendar`
    Returns:
      the week year represented by this `GregorianCalendar`. If the `ERA` value is `BC`, the year is represented by 0 or a negative number: BC 1 is 0, BC 2 is -1, BC 3 is -2, and so on.
    Throws:
      `IllegalArgumentException` - if any of the calendar fields is invalid in non-lenient mode.
    Since:
      1.7
    See Also:
      - `isWeekDateSupported()`
      - `getWeeksInWeekYear()`
      - `Calendar.getFirstDayOfWeek()`
      - `Calendar.getMinimalDaysInFirstWeek()`

  - ### setWeekDate

    public void setWeekDate(int weekYear, int weekOfYear, int dayOfWeek)
    Sets this `GregorianCalendar` to the date given by the date specifiers - `weekYear`, `weekOfYear`, and `dayOfWeek`. `weekOfYear` follows the `WEEK_OF_YEAR` numbering. The `dayOfWeek` value must be one of the `DAY_OF_WEEK` values: `SUNDAY` to `SATURDAY`.

    Note that the numeric day-of-week representation differs from the ISO 8601 standard, and that the `weekOfYear` numbering is compatible with the standard when `getFirstDayOfWeek()` is `MONDAY` and `getMinimalDaysInFirstWeek()` is 4.

    Unlike the `set` method, all of the calendar fields and the instant of time value are calculated upon return.

    If `weekOfYear` is out of the valid week-of-year range in `weekYear`, the `weekYear` and `weekOfYear` values are adjusted in lenient mode, or an `IllegalArgumentException` is thrown in non-lenient mode.

    Overrides:
      `setWeekDate` in class `Calendar`
    Parameters:
      `weekYear` - the week year
      `weekOfYear` - the week number based on `weekYear`
      `dayOfWeek` - the day of week value: one of the constants for the `DAY_OF_WEEK` field: `SUNDAY`, ..., `SATURDAY`.
    Throws:
      `IllegalArgumentException` - if any of the given date specifiers is invalid, or if any of the calendar fields are inconsistent with the given date specifiers in non-lenient mode
    Since:
      1.7
    See Also:
      - `isWeekDateSupported()`
      - `Calendar.getFirstDayOfWeek()`
      - `Calendar.getMinimalDaysInFirstWeek()`

  - ### getWeeksInWeekYear

    public int getWeeksInWeekYear()
    Returns the number of weeks in the week year represented by this `GregorianCalendar`.

    For example, if this `GregorianCalendar`'s date is December 31, 2008 with the ISO 8601 compatible setting, this method will return 53 for the period: December 29, 2008 to January 3, 2010 while `getActualMaximum(WEEK_OF_YEAR)` will return 52 for the period: December 31, 2007 to December 28, 2008.

    Overrides:
      `getWeeksInWeekYear` in class `Calendar`
    Returns:
      the number of weeks in the week year.
    Since:
      1.7
    See Also:
      - `Calendar.WEEK_OF_YEAR`
      - `getWeekYear()`
      - `getActualMaximum(int)`

  - ### computeFields

    protected void computeFields()
    Converts the time value (millisecond offset from the Epoch) to calendar field values. The time is not recomputed first; to recompute the time, then the fields, call the `complete` method.
    Specified by:
      `computeFields` in class `Calendar`
    See Also:
      - `Calendar.complete()`

  - ### computeTime

    protected void computeTime()
    Converts calendar field values to the time value (millisecond offset from the Epoch).
    Specified by:
      `computeTime` in class `Calendar`
    Throws:
      `IllegalArgumentException` - if any calendar fields are invalid.
    See Also:
      - `Calendar.complete()`
      - `Calendar.computeFields()`

  - ### toZonedDateTime

    public ZonedDateTime toZonedDateTime()
    Converts this object to a `ZonedDateTime` that represents the same point on the time-line as this `GregorianCalendar`.

    Since this object supports a Julian-Gregorian cutover date and `ZonedDateTime` does not, it is possible that the resulting year, month and day will have different values. The result will represent the correct date in the ISO calendar system, which will also be the same value for Modified Julian Days.

    Returns:
      a zoned date-time representing the same point on the time-line as this gregorian calendar
    Since:
      1.8

  - ### from

    public static GregorianCalendar from(ZonedDateTime zdt)
    Obtains an instance of `GregorianCalendar` with the default locale from a `ZonedDateTime` object.

    Since `ZonedDateTime` does not support a Julian-Gregorian cutover date and uses ISO calendar system, the return GregorianCalendar is a pure Gregorian calendar and uses ISO 8601 standard for week definitions, which has `MONDAY` as the `FirstDayOfWeek` and `4` as the value of the `MinimalDaysInFirstWeek`.

    `ZoneDateTime` can store points on the time-line further in the future and further in the past than `GregorianCalendar`. In this scenario, this method will throw an `IllegalArgumentException` exception.

    Parameters:
      `zdt` - the zoned date-time object to convert
    Returns:
      the gregorian calendar representing the same point on the time-line as the zoned date-time provided
    Throws:
      `NullPointerException` - if `zdt` is null
      `IllegalArgumentException` - if the zoned date-time is too large to represent as a `GregorianCalendar`
    Since:
      1.8