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# A RFC 2445 Excerpt: Recurrence

This appendix contains an excerpt of RFC 2445, "Internet Calendaring and Scheduling Core Object Specification (ICalendar)." Section 4.3.10 of this RFC is used as the basis for iCalendar recurrence schedules. This appendix provides a listing of Section 4.3.10, and highlights specific areas that are not supported. Some examples are also provided.

## A.1 RFC 2445, Section 4.3.10. Recurrence Rule

This section provides a listing of RFC 2445, Section 4.3.10.

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    4.3.10 Recurrence Rule

       Value Name: RECUR

       Purpose: This value type is used to identify properties that contain
       a recurrence rule specification.

       Formal Definition: The value type is defined by the following
       notation:

         recur      = "FREQ"=freq *(

                    ; either UNTIL or COUNT may appear in a 'recur',
                    ; but UNTIL and COUNT MUST NOT occur in the same 'recur'

                    ( ";" "UNTIL" "=" enddate ) /
                    ( ";" "COUNT" "=" 1*DIGIT ) /

                    ; the rest of these keywords are optional,
                    ; but MUST NOT occur more than once

                    ( ";" "INTERVAL" "=" 1*DIGIT )          /
                    ( ";" "BYSECOND" "=" byseclist )        /
                    ( ";" "BYMINUTE" "=" byminlist )        /
                    ( ";" "BYHOUR" "=" byhrlist )           /
                    ( ";" "BYDAY" "=" bywdaylist )          /
                    ( ";" "BYMONTHDAY" "=" bymodaylist )    /
                    ( ";" "BYYEARDAY" "=" byyrdaylist )     /
                    ( ";" "BYWEEKNO" "=" bywknolist )       /
                    ( ";" "BYMONTH" "=" bymolist )          /
                    ( ";" "BYSETPOS" "=" bysplist )         /
                    ( ";" "WKST" "=" weekday )              /
                    ( ";" x-name "=" text )
                    )

         freq       = "SECONDLY" / "MINUTELY" / "HOURLY" / "DAILY"
                    / "WEEKLY" / "MONTHLY" / "YEARLY"

         enddate    = date
         enddate    =/ date-time            ;An UTC value

         byseclist  = seconds / ( seconds *("," seconds) )

         seconds    = 1DIGIT / 2DIGIT       ;0 to 59

         byminlist  = minutes / ( minutes *("," minutes) )

         minutes    = 1DIGIT / 2DIGIT       ;0 to 59

         byhrlist   = hour / ( hour *("," hour) )

         hour       = 1DIGIT / 2DIGIT       ;0 to 23

         bywdaylist = weekdaynum / ( weekdaynum *("," weekdaynum) )

         weekdaynum = [([plus] ordwk / minus ordwk)] weekday

         plus       = "+"

         minus      = "-"

         ordwk      = 1DIGIT / 2DIGIT       ;1 to 53

         weekday    = "SU" / "MO" / "TU" / "WE" / "TH" / "FR" / "SA"
         ;Corresponding to SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY,
         ;FRIDAY, SATURDAY and SUNDAY days of the week.

         bymodaylist = monthdaynum / ( monthdaynum *("," monthdaynum) )

         monthdaynum = ([plus] ordmoday) / (minus ordmoday)

         ordmoday   = 1DIGIT / 2DIGIT       ;1 to 31

         byyrdaylist = yeardaynum / ( yeardaynum *("," yeardaynum) )

         yeardaynum = ([plus] ordyrday) / (minus ordyrday)

         ordyrday   = 1DIGIT / 2DIGIT / 3DIGIT      ;1 to 366

         bywknolist = weeknum / ( weeknum *("," weeknum) )

         weeknum    = ([plus] ordwk) / (minus ordwk)

         bymolist   = monthnum / ( monthnum *("," monthnum) )

         monthnum   = 1DIGIT / 2DIGIT       ;1 to 12

         bysplist   = setposday / ( setposday *("," setposday) )

         setposday  = yeardaynum

       Description: If the property permits, multiple "recur" values are
       specified by a COMMA character (US-ASCII decimal 44) separated list
       of values. The value type is a structured value consisting of a list
       of one or more recurrence grammar parts. Each rule part is defined by
       a NAME=VALUE pair. The rule parts are separated from each other by
       the SEMICOLON character (US-ASCII decimal 59). The rule parts are not
       ordered in any particular sequence. Individual rule parts MUST only
       be specified once.

       The FREQ rule part identifies the type of recurrence rule. This rule
       part MUST be specified in the recurrence rule. Valid values include
       SECONDLY, to specify repeating events based on an interval of a
       second or more; MINUTELY, to specify repeating events based on an
       interval of a minute or more; HOURLY, to specify repeating events
       based on an interval of an hour or more; DAILY, to specify repeating
       events based on an interval of a day or more; WEEKLY, to specify
       repeating events based on an interval of a week or more; MONTHLY, to
       specify repeating events based on an interval of a month or more; and
       YEARLY, to specify repeating events based on an interval of a year or
       more.

       The INTERVAL rule part contains a positive integer representing how
       often the recurrence rule repeats. The default value is "1", meaning
       every second for a SECONDLY rule, or every minute for a MINUTELY
       rule, every hour for an HOURLY rule, every day for a DAILY rule,
       every week for a WEEKLY rule, every month for a MONTHLY rule and
       every year for a YEARLY rule.

       The UNTIL rule part defines a date-time value which bounds the
       recurrence rule in an inclusive manner. If the value specified by
       UNTIL is synchronized with the specified recurrence, this date or
       date-time becomes the last instance of the recurrence. If specified
       as a date-time value, then it MUST be specified in an UTC time
       format. If not present, and the COUNT rule part is also not present,
       the RRULE is considered to repeat forever.

       The COUNT rule part defines the number of occurrences at which to
       range-bound the recurrence. The "DTSTART" property value, if
       specified, counts as the first occurrence.

       The BYSECOND rule part specifies a COMMA character (US-ASCII decimal
       44) separated list of seconds within a minute. Valid values are 0 to
       59. The BYMINUTE rule part specifies a COMMA character (US-ASCII
       decimal 44) separated list of minutes within an hour. Valid values
       are 0 to 59. The BYHOUR rule part specifies a COMMA character (US-
       ASCII decimal 44) separated list of hours of the day. Valid values
       are 0 to 23.

       The BYDAY rule part specifies a COMMA character (US-ASCII decimal 44)
       separated list of days of the week; MO indicates Monday; TU indicates
       Tuesday; WE indicates Wednesday; TH indicates Thursday; FR indicates
       Friday; SA indicates Saturday; SU indicates Sunday.

       Each BYDAY value can also be preceded by a positive (+n) or negative
       (-n) integer. If present, this indicates the nth occurrence of the
       specific day within the MONTHLY or YEARLY RRULE. For example, within
       a MONTHLY rule, +1MO (or simply 1MO) represents the first Monday
       within the month, whereas -1MO represents the last Monday of the
       month. If an integer modifier is not present, it means all days of
       this type within the specified frequency. For example, within a
       MONTHLY rule, MO represents all Mondays within the month.

       The BYMONTHDAY rule part specifies a COMMA character (ASCII decimal
       44) separated list of days of the month. Valid values are 1 to 31 or
       -31 to -1. For example, -10 represents the tenth to the last day of
       the month.

       The BYYEARDAY rule part specifies a COMMA character (US-ASCII decimal
       44) separated list of days of the year. Valid values are 1 to 366 or
       -366 to -1. For example, -1 represents the last day of the year
       (December 31st) and -306 represents the 306th to the last day of the
       year (March 1st).

       The BYWEEKNO rule part specifies a COMMA character (US-ASCII decimal
       44) separated list of ordinals specifying weeks of the year. Valid
       values are 1 to 53 or -53 to -1. This corresponds to weeks according
       to week numbering as defined in [ISO 8601]. A week is defined as a
       seven day period, starting on the day of the week defined to be the
       week start (see WKST). Week number one of the calendar year is the
       first week which contains at least four (4) days in that calendar
       year. This rule part is only valid for YEARLY rules. For example, 3
       represents the third week of the year.

            Note: Assuming a Monday week start, week 53 can only occur when
            Thursday is January 1 or if it is a leap year and Wednesday is
            January 1.

       The BYMONTH rule part specifies a COMMA character (US-ASCII decimal
       44) separated list of months of the year. Valid values are 1 to 12.

       The WKST rule part specifies the day on which the workweek starts.
       Valid values are MO, TU, WE, TH, FR, SA and SU. This is significant
       when a WEEKLY RRULE has an interval greater than 1, and a BYDAY rule
       part is specified. This is also significant when in a YEARLY RRULE
       when a BYWEEKNO rule part is specified. The default value is MO.

       The BYSETPOS rule part specifies a COMMA character (US-ASCII decimal
       44) separated list of values which corresponds to the nth occurrence
       within the set of events specified by the rule. Valid values are 1 to
       366 or -366 to -1. It MUST only be used in conjunction with another
       BYxxx rule part. For example "the last work day of the month" could
       be represented as:

         RRULE:FREQ=MONTHLY;BYDAY=MO,TU,WE,TH,FR;BYSETPOS=-1

       Each BYSETPOS value can include a positive (+n) or negative (-n)
       integer. If present, this indicates the nth occurrence of the
       specific occurrence within the set of events specified by the rule.

       If BYxxx rule part values are found which are beyond the available
       scope (ie, BYMONTHDAY=30 in February), they are simply ignored.

       Information, not contained in the rule, necessary to determine the
       various recurrence instance start time and dates are derived from the
       Start Time (DTSTART) entry attribute. For example,
       "FREQ=YEARLY;BYMONTH=1" doesn't specify a specific day within the
       month or a time. This information would be the same as what is
       specified for DTSTART.

       BYxxx rule parts modify the recurrence in some manner. BYxxx rule
       parts for a period of time which is the same or greater than the
       frequency generally reduce or limit the number of occurrences of the
       recurrence generated. For example, "FREQ=DAILY;BYMONTH=1" reduces the
       number of recurrence instances from all days (if BYMONTH tag is not
       present) to all days in January. BYxxx rule parts for a period of
       time less than the frequency generally increase or expand the number
       of occurrences of the recurrence. For example,
       "FREQ=YEARLY;BYMONTH=1,2" increases the number of days within the
       yearly recurrence set from 1 (if BYMONTH tag is not present) to 2.

       If multiple BYxxx rule parts are specified, then after evaluating the
       specified FREQ and INTERVAL rule parts, the BYxxx rule parts are
       applied to the current set of evaluated occurrences in the following
       order: BYMONTH, BYWEEKNO, BYYEARDAY, BYMONTHDAY, BYDAY, BYHOUR,
       BYMINUTE, BYSECOND and BYSETPOS; then COUNT and UNTIL are evaluated.

       Here is an example of evaluating multiple BYxxx rule parts.

         DTSTART;TZID=US-Eastern:19970105T083000
         RRULE:FREQ=YEARLY;INTERVAL=2;BYMONTH=1;BYDAY=SU;BYHOUR=8,9;
          BYMINUTE=30

       First, the "INTERVAL=2" would be applied to "FREQ=YEARLY" to arrive
       at "every other year". Then, "BYMONTH=1" would be applied to arrive
       at "every January, every other year". Then, "BYDAY=SU" would be
       applied to arrive at "every Sunday in January, every other year".
       Then, "BYHOUR=8,9" would be applied to arrive at "every Sunday in
       January at 8 AM and 9 AM, every other year". Then, "BYMINUTE=30"
       would be applied to arrive at "every Sunday in January at 8:30 AM and
       9:30 AM, every other year". Then, lacking information from RRULE, the
       second is derived from DTSTART, to end up in "every Sunday in January
       at 8:30:00 AM and 9:30:00 AM, every other year". Similarly, if the
       BYMINUTE, BYHOUR, BYDAY, BYMONTHDAY or BYMONTH rule part were
       missing, the appropriate minute, hour, day or month would have been
       retrieved from the "DTSTART" property.

       No additional content value encoding (i.e., BACKSLASH character
       encoding) is defined for this value type.

       Example: The following is a rule which specifies 10 meetings which
       occur every other day:

         FREQ=DAILY;COUNT=10;INTERVAL=2

       There are other examples specified in the "RRULE" specification.
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    11.  Full Copyright Statement

       Copyright (C) The Internet Society (1998).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

## A.2 Job Scheduler Implementation of the Recurrence Rule

Oracle Application Server Containers for J2EE implements the recurrence rule of RFC 2445 as follows:

- The UNTIL rule is not supported. Similar functionality can be achieved by using the `setEndDate()` method of the `recurSchedule` object.

- The COUNT rule is not supported. Similar functionality can be achieved by using the `setCount()` method of the `recurSchedule` object.

- The BYDAY clause supports both two- and three-letter abbreviations for days of the week (for example, either `MO` or `MON` may be used to represent Monday).

- The BYSETPOS rule is not supported.

- The WKST rule is not supported.

## A.3 RFC 2445, Section 4.8.5.4. Recurrence Rule Examples

This section provides a listing of RFC 2445, Section 4.8.5.4. This section contains many examples which you might find helpful.

Note: The examples that use the BYSETPOS and WKST rules are omitted, because these rules are not supported by Oracle Application Server Containers for J2EE.

    4.8.5.4 Recurrence Rule

       Property Name: RRULE

       Purpose: This property defines a rule or repeating pattern for
       recurring events, to-dos, or time zone definitions.

       Value Type: RECUR

       Property Parameters: Non-standard property parameters can be
       specified on this property.

       Conformance: This property can be specified one or more times in
       recurring "VEVENT", "VTODO" and "VJOURNAL" calendar components. It
       can also be specified once in each STANDARD or DAYLIGHT sub-component
       of the "VTIMEZONE" calendar component.

       Description: The recurrence rule, if specified, is used in computing
       the recurrence set. The recurrence set is the complete set of
       recurrence instances for a calendar component. The recurrence set is
       generated by considering the initial "DTSTART" property along with
       the "RRULE", "RDATE", "EXDATE" and "EXRULE" properties contained
       within the iCalendar object. The "DTSTART" property defines the first
       instance in the recurrence set. Multiple instances of the "RRULE" and
       "EXRULE" properties can also be specified to define more
       sophisticated recurrence sets. The final recurrence set is generated
       by gathering all of the start date/times generated by any of the
       specified "RRULE" and "RDATE" properties, and excluding any start
       date/times which fall within the union of start date/times generated
       by any specified "EXRULE" and "EXDATE" properties. This implies that
       start date/times within exclusion related properties (i.e., "EXDATE"
       and "EXRULE") take precedence over those specified by inclusion
       properties (i.e., "RDATE" and "RRULE"). Where duplicate instances are
       generated by the "RRULE" and "RDATE" properties, only one recurrence
       is considered. Duplicate instances are ignored.

       The "DTSTART" and "DTEND" property pair or "DTSTART" and "DURATION"
       property pair, specified within the iCalendar object defines the
       first instance of the recurrence. When used with a recurrence rule,
       the "DTSTART" and "DTEND" properties MUST be specified in local time
       and the appropriate set of "VTIMEZONE" calendar components MUST be
       included. For detail on the usage of the "VTIMEZONE" calendar
       component, see the "VTIMEZONE" calendar component definition.

       Any duration associated with the iCalendar object applies to all
       members of the generated recurrence set. Any modified duration for
       specific recurrences MUST be explicitly specified using the "RDATE"
       property.

       Format Definition: This property is defined by the following
       notation:

         rrule      = "RRULE" rrulparam ":" recur CRLF

         rrulparam  = *(";" xparam)

       Example: All examples assume the Eastern United States time zone.

       Daily for 10 occurrences:

         DTSTART;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=DAILY;COUNT=10

         ==> (1997 9:00 AM EDT)September 2-11

       Daily until December 24, 1997:

         DTSTART;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=DAILY;UNTIL=19971224T000000Z

         ==> (1997 9:00 AM EDT)September 2-30;October 1-25
             (1997 9:00 AM EST)October 26-31;November 1-30;December 1-23

       Every other day - forever:

         DTSTART;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=DAILY;INTERVAL=2
         ==> (1997 9:00 AM EDT)September2,4,6,8...24,26,28,30;
              October 2,4,6...20,22,24
             (1997 9:00 AM EST)October 26,28,30;November 1,3,5,7...25,27,29;
              Dec 1,3,...

       Every 10 days, 5 occurrences:

         DTSTART;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=DAILY;INTERVAL=10;COUNT=5

         ==> (1997 9:00 AM EDT)September 2,12,22;October 2,12

       Everyday in January, for 3 years:

         DTSTART;TZID=US-Eastern:19980101T090000
         RRULE:FREQ=YEARLY;UNTIL=20000131T090000Z;
          BYMONTH=1;BYDAY=SU,MO,TU,WE,TH,FR,SA
         or
         RRULE:FREQ=DAILY;UNTIL=20000131T090000Z;BYMONTH=1

         ==> (1998 9:00 AM EDT)January 1-31
             (1999 9:00 AM EDT)January 1-31
             (2000 9:00 AM EDT)January 1-31

       Weekly for 10 occurrences

         DTSTART;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=WEEKLY;COUNT=10

         ==> (1997 9:00 AM EDT)September 2,9,16,23,30;October 7,14,21
             (1997 9:00 AM EST)October 28;November 4

       Weekly until December 24, 1997

         DTSTART;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=WEEKLY;UNTIL=19971224T000000Z

         ==> (1997 9:00 AM EDT)September 2,9,16,23,30;October 7,14,21
             (1997 9:00 AM EST)October 28;November 4,11,18,25;
                               December 2,9,16,23
       Monthly on the 1st Friday for ten occurrences:

         DTSTART;TZID=US-Eastern:19970905T090000
         RRULE:FREQ=MONTHLY;COUNT=10;BYDAY=1FR

         ==> (1997 9:00 AM EDT)September 5;October 3
             (1997 9:00 AM EST)November 7;Dec 5
             (1998 9:00 AM EST)January 2;February 6;March 6;April 3
             (1998 9:00 AM EDT)May 1;June 5

       Monthly on the 1st Friday until December 24, 1997:

         DTSTART;TZID=US-Eastern:19970905T090000
         RRULE:FREQ=MONTHLY;UNTIL=19971224T000000Z;BYDAY=1FR

         ==> (1997 9:00 AM EDT)September 5;October 3
             (1997 9:00 AM EST)November 7;December 5

       Every other month on the 1st and last Sunday of the month for 10
       occurrences:

         DTSTART;TZID=US-Eastern:19970907T090000
         RRULE:FREQ=MONTHLY;INTERVAL=2;COUNT=10;BYDAY=1SU,-1SU

         ==> (1997 9:00 AM EDT)September 7,28
             (1997 9:00 AM EST)November 2,30
             (1998 9:00 AM EST)January 4,25;March 1,29
             (1998 9:00 AM EDT)May 3,31

       Monthly on the second to last Monday of the month for 6 months:

         DTSTART;TZID=US-Eastern:19970922T090000
         RRULE:FREQ=MONTHLY;COUNT=6;BYDAY=-2MO

         ==> (1997 9:00 AM EDT)September 22;October 20
             (1997 9:00 AM EST)November 17;December 22
             (1998 9:00 AM EST)January 19;February 16

       Monthly on the third to the last day of the month, forever:

         DTSTART;TZID=US-Eastern:19970928T090000
         RRULE:FREQ=MONTHLY;BYMONTHDAY=-3

         ==> (1997 9:00 AM EDT)September 28
             (1997 9:00 AM EST)October 29;November 28;December 29
             (1998 9:00 AM EST)January 29;February 26
         ...

       Monthly on the 2nd and 15th of the month for 10 occurrences:

         DTSTART;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=MONTHLY;COUNT=10;BYMONTHDAY=2,15

         ==> (1997 9:00 AM EDT)September 2,15;October 2,15
             (1997 9:00 AM EST)November 2,15;December 2,15
             (1998 9:00 AM EST)January 2,15

       Monthly on the first and last day of the month for 10 occurrences:

         DTSTART;TZID=US-Eastern:19970930T090000
         RRULE:FREQ=MONTHLY;COUNT=10;BYMONTHDAY=1,-1

         ==> (1997 9:00 AM EDT)September 30;October 1
             (1997 9:00 AM EST)October 31;November 1,30;December 1,31
             (1998 9:00 AM EST)January 1,31;February 1

       Every 18 months on the 10th thru 15th of the month for 10
       occurrences:

         DTSTART;TZID=US-Eastern:19970910T090000
         RRULE:FREQ=MONTHLY;INTERVAL=18;COUNT=10;BYMONTHDAY=10,11,12,13,14,
          15

         ==> (1997 9:00 AM EDT)September 10,11,12,13,14,15
             (1999 9:00 AM EST)March 10,11,12,13

       Every Tuesday, every other month:

         DTSTART;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=MONTHLY;INTERVAL=2;BYDAY=TU

         ==> (1997 9:00 AM EDT)September 2,9,16,23,30
             (1997 9:00 AM EST)November 4,11,18,25
             (1998 9:00 AM EST)January 6,13,20,27;March 3,10,17,24,31
         ...

       Yearly in June and July for 10 occurrences:

         DTSTART;TZID=US-Eastern:19970610T090000
         RRULE:FREQ=YEARLY;COUNT=10;BYMONTH=6,7
         ==> (1997 9:00 AM EDT)June 10;July 10
             (1998 9:00 AM EDT)June 10;July 10
             (1999 9:00 AM EDT)June 10;July 10
             (2000 9:00 AM EDT)June 10;July 10
             (2001 9:00 AM EDT)June 10;July 10
         Note: Since none of the BYDAY, BYMONTHDAY or BYYEARDAY components
         are specified, the day is gotten from DTSTART

       Every other year on January, February, and March for 10 occurrences:

         DTSTART;TZID=US-Eastern:19970310T090000
         RRULE:FREQ=YEARLY;INTERVAL=2;COUNT=10;BYMONTH=1,2,3

         ==> (1997 9:00 AM EST)March 10
             (1999 9:00 AM EST)January 10;February 10;March 10
             (2001 9:00 AM EST)January 10;February 10;March 10
             (2003 9:00 AM EST)January 10;February 10;March 10

       Every 3rd year on the 1st, 100th and 200th day for 10 occurrences:

         DTSTART;TZID=US-Eastern:19970101T090000
         RRULE:FREQ=YEARLY;INTERVAL=3;COUNT=10;BYYEARDAY=1,100,200

         ==> (1997 9:00 AM EST)January 1
             (1997 9:00 AM EDT)April 10;July 19
             (2000 9:00 AM EST)January 1
             (2000 9:00 AM EDT)April 9;July 18
             (2003 9:00 AM EST)January 1
             (2003 9:00 AM EDT)April 10;July 19
             (2006 9:00 AM EST)January 1

       Every 20th Monday of the year, forever:

         DTSTART;TZID=US-Eastern:19970519T090000
         RRULE:FREQ=YEARLY;BYDAY=20MO

         ==> (1997 9:00 AM EDT)May 19
             (1998 9:00 AM EDT)May 18
             (1999 9:00 AM EDT)May 17
         ...

       Monday of week number 20 (where the default start of the week is
       Monday), forever:

         DTSTART;TZID=US-Eastern:19970512T090000
         RRULE:FREQ=YEARLY;BYWEEKNO=20;BYDAY=MO

         ==> (1997 9:00 AM EDT)May 12
             (1998 9:00 AM EDT)May 11
             (1999 9:00 AM EDT)May 17
         ...

       Every Thursday in March, forever:

         DTSTART;TZID=US-Eastern:19970313T090000
         RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=TH

         ==> (1997 9:00 AM EST)March 13,20,27
             (1998 9:00 AM EST)March 5,12,19,26
             (1999 9:00 AM EST)March 4,11,18,25
         ...

       Every Thursday, but only during June, July, and August, forever:

         DTSTART;TZID=US-Eastern:19970605T090000
         RRULE:FREQ=YEARLY;BYDAY=TH;BYMONTH=6,7,8

         ==> (1997 9:00 AM EDT)June 5,12,19,26;July 3,10,17,24,31;
                           August 7,14,21,28
             (1998 9:00 AM EDT)June 4,11,18,25;July 2,9,16,23,30;
                           August 6,13,20,27
             (1999 9:00 AM EDT)June 3,10,17,24;July 1,8,15,22,29;
                           August 5,12,19,26
         ...

       Every Friday the 13th, forever:

         DTSTART;TZID=US-Eastern:19970902T090000
         EXDATE;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=MONTHLY;BYDAY=FR;BYMONTHDAY=13

         ==> (1998 9:00 AM EST)February 13;March 13;November 13
             (1999 9:00 AM EDT)August 13
             (2000 9:00 AM EDT)October 13
         ...

       The first Saturday that follows the first Sunday of the month,
        forever:

         DTSTART;TZID=US-Eastern:19970913T090000
         RRULE:FREQ=MONTHLY;BYDAY=SA;BYMONTHDAY=7,8,9,10,11,12,13

         ==> (1997 9:00 AM EDT)September 13;October 11
             (1997 9:00 AM EST)November 8;December 13
             (1998 9:00 AM EST)January 10;February 7;March 7
             (1998 9:00 AM EDT)April 11;May 9;June 13...
         ...

       Every four years, the first Tuesday after a Monday in November,
       forever (U.S. Presidential Election day):

         DTSTART;TZID=US-Eastern:19961105T090000
         RRULE:FREQ=YEARLY;INTERVAL=4;BYMONTH=11;BYDAY=TU;BYMONTHDAY=2,3,4,
          5,6,7,8

         ==> (1996 9:00 AM EST)November 5
             (2000 9:00 AM EST)November 7
             (2004 9:00 AM EST)November 2
         ...

       Every 3 hours from 9:00 AM to 5:00 PM on a specific day:

         DTSTART;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=HOURLY;INTERVAL=3;UNTIL=19970902T170000Z

         ==> (September 2, 1997 EDT)09:00,12:00,15:00

       Every 15 minutes for 6 occurrences:

         DTSTART;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=MINUTELY;INTERVAL=15;COUNT=6

         ==> (September 2, 1997 EDT)09:00,09:15,09:30,09:45,10:00,10:15

       Every hour and a half for 4 occurrences:

         DTSTART;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=MINUTELY;INTERVAL=90;COUNT=4

         ==> (September 2, 1997 EDT)09:00,10:30;12:00;13:30

       Every 20 minutes from 9:00 AM to 4:40 PM every day:

         DTSTART;TZID=US-Eastern:19970902T090000
         RRULE:FREQ=DAILY;BYHOUR=9,10,11,12,13,14,15,16;BYMINUTE=0,20,40
         or
         RRULE:FREQ=MINUTELY;INTERVAL=20;BYHOUR=9,10,11,12,13,14,15,16

         ==> (September 2, 1997 EDT)9:00,9:20,9:40,10:00,10:20,
                                    ... 16:00,16:20,16:40
             (September 3, 1997 EDT)9:00,9:20,9:40,10:00,10:20,
                                   ...16:00,16:20,16:40
         ...