Support ISO 8601 Durations and Periods, following RFC 3339's Appendix A

[gustavorps]

Durations:

   dur-second        = 1*DIGIT "S"
   dur-minute        = 1*DIGIT "M" [dur-second]
   dur-hour          = 1*DIGIT "H" [dur-minute]
   dur-time          = "T" (dur-hour / dur-minute / dur-second)
   dur-day           = 1*DIGIT "D"
   dur-week          = 1*DIGIT "W"
   dur-month         = 1*DIGIT "M" [dur-day]
   dur-year          = 1*DIGIT "Y" [dur-month]
   dur-date          = (dur-day / dur-month / dur-year) [dur-time]

   duration          = "P" (dur-date / dur-time / dur-week)



Klyne, et. al.              Standards Track                    [Page 13]

RFC 3339       Date and Time on the Internet: Timestamps       July 2002


Periods:

   period-explicit   = iso-date-time "/" iso-date-time
   period-start      = iso-date-time "/" duration
   period-end        = duration "/" iso-date-time

   period            = period-explicit / period-start / period-end

[tonyg]

These definitions come from RFC3339 Appendix A; I don't think they're normative. However, they could be a useful addition. Perhaps separate modules, rfc3339.duration and rfc3339.period. I imagine durations could be represented with Python's timedelta; but there's no obvious existing candidate for periods.