cron recurring ("man 5 crontab")
     parseable using http://search.cpan.org/~msisk/DateTime-Event-Cron-0.07/lib/DateTime/Event/Cron.pm
   no duration just recurrence and start.
   5 fields (plus rest of line for command)
   minute,hour,monthday,month,weekday
   each can be a numeric range (like 0-59), a specific number, or "*" which is the maximal range
   a range means do it at all matching points in the range
   to step within the range, follow by "/" and digit.
   "*/2" means every 2 hours

iCalendar recurring (RFC2445 section 4.8.5)
  some debate about its usefulness: http://lists.osafoundation.org/pipermail/ietf-calsify/2004-August/000011.html
  DSTART - the starting date of the event as usual.
  RRULE = recurrence rule.
    either (start,end) or (start,duration,rrule)
    RRULE:FREQ=WEEKLY;UNTIL=20021214T055959;INTERVAL=1;BYDAY=MO,TU,WE,TH
    FREQ is mandatory
    to end: can have UNTIL, COUNT, or none
    INTERVAL is optional, means how many of the units in FREQ (default 1)
    BY* (except BYSETPOS) lists the particular times within that frequency unit.
      BYDAY is suitable for FREQ=WEEKLY.
      BYMONTH and BYWEEKNO are suitable for FREQ=YEARLY
      first friday each month, for 10 months:
         RRULE:FREQ=MONTHLY;COUNT=10;BYDAY=1FR
      Every other month on the 1st and last Sunday of the month for 10 occurrences:
         RRULE:FREQ=MONTHLY;INTERVAL=2;COUNT=10;BYDAY=1SU,-1SU
      Monthly on the third to the last day of the month, forever
         RRULE:FREQ=MONTHLY;BYMONTHDAY=-3
      Every 20th Monday of the year, forever
         RRULE:FREQ=YEARLY;BYDAY=20MO
      Every Friday the 13th, forever
         RRULE:FREQ=MONTHLY;BYDAY=FR;BYMONTHDAY=13
    BYSETPOS only allowed when another BY* is specified
      it selects among the set specified by the BY* option
      a -1 means the last in the set; a 1 means the first.
      thus this is last weekday in each month: 
         RRULE:FREQ=MONTHLY;BYDAY=MO,TU,WE,TH,FR;BYSETPOS=-1
      this is first day of each year where the month start falls on a monday (????):
         RRULE:FREQ=YEARLY;BYDAY=MO;BYMONTHDAY=1;BYSETPOS=1
      Note that BYSETPOS is not always necessary, because you can say:
         RRULE:FREQ=MONTHLY;INTERVAL=1;BYDAY=-1TH
      For last thursday of each month
    WKST
      week start. relevant for FREQ=WEEKLY
    In general:
      start with set of FREQ, starting with DTSTART inclusive (DTSTART is mandatory for RRULE but not RDATE)
         and if either DTEND or UNTIL is specified, restrict to lesser (inclusive)
      skip by INTERVAL (default 1 meaning no skipping)
      limit by BYDAY/BYMONTH/BYWEEKNO/BYMONTHDAY/BYMINUTE/BYHOUR/BYSECOND
      limit by BYSETPOS (for each BY* subset within a freq)
      limit by COUNT
      

    DSTART supplies missing info. but can specify there.
      FREQ=WEEKLY;BYDAY=SU;BYHOUR=8;BYMINUTE=30 means every sunday at 8:30am
  EXRULE = rule out things in RRULE
    EXRULE:weekly on Tuesdays
    RRULE:daily
    means every day except tuesdays
  RDATE = list of specific dates/times
  XDATE = rule out things in RDATE
  RECUR
    can have multiple RRULE, EXRULE within a single RECUR (all share same DTSTART)
    union of RRULE,RDATE, minus union oF EXRULE,XDATE

ISO has recurrence, periods, duration.
    http://www.mcs.vuw.ac.nz/technical/software/SGML/doc/iso8601/ISO8601.html
    http://www.cs.tut.fi/~jkorpela/iso8601.html
    draft copies from: http://www.astroclark.freeserve.co.uk/iso8601/index.html
    http://dmoz.org/Science/Reference/Standards/Individual_Standards/ISO_8601/
    http://www.webaugur.com/bibliotheca/standards/iso8601/ISO-FDIS-8601/index-29.html
   week date (5.2.3)
     "-W-5" means the 5th day of the implied week
     "-W155" means the 5th day of the 15th week of the implied year
     day 1 of a week is Monday, and day 7 is Sunday
   duration has leading P
     "P18Y9M4DT11H9M8S"
     "P2W" [2 weeks]
   period (aka "time interval") can be start and end separated by slash
     "19930214T131030/19930214T131031"
     1998-05-12T14:15Z/1998-05-13T16:00Z
     1998-05-12T14:15Z/16:00Z
     1998-05-12/15
     [end is same precision as start, and can omit higher order parts]
   period can be start and duration
     "19930214T131030/P18Y9M4DT11H9M8S"
   period can be duration and end
     "P18Y9M4DT11H9M8S/19930214T131030"
   recurrence has: "R" [number of recurrences] "/" timeinterval
     so just assumes that the timeintervals are adjoining (duration is used interval of recurrence, nothing about duration of that event)
     not sufficient for repetitive events: http://groups.yahoo.com/group/ISO8601/message/125
     "R8/P1Y" means 8 years. "R6/P15W" means 6 periods of 15weeks.
     "R4/1999-12-31/2000-12-31 " means 4 years starting in new years eve

XML Schema
    http://www.w3.org/TR/xmlschema-2/ does not define a recurring format
    it does have duration: http://www.w3.org/TR/xmlschema-2/#duration
       but does not constraint integer values
       P1Y2M3DT10H30M
       -P120D