2007-03-01T000000Z periodType The type of period specified. A "period" can specify a duration and/or a start time and/or an end time. All times are in the ISO 8601 format. Remember that the XML times can specify the time range for years, months, days, hours, minutes, seconds, and partial seconds. It may per possible to have two periodTypes defined - one for the combination of start/end/duration and a second for enhancing the frequency. Cycle: The time range for the repetitive values may specify a start time indicating the repeat reference time, and may additionally specify a duration indicating cycle validity (i.e. implied "start" or "durationStart" values). Cycle: Certain periods, like "semi-monthly" (i.e. twice a month) cannot be expressed using ISO durations. Thus, the "periodType" fills in where necessary. absolute A start and end time are provided with no calculation required duration Only a duration is provided durationEnd An end and duration are provided indicating an end of time period back through the start time calculated durationStart A start and duration are provided indicating a beginning time up through the end time calculated end Only an end time is provided full All three values (end, start, duration) are provided so any may be used (they should all crosscheck) start Only a start time is provided dateTime The "start" time specifics a single date - there is no period or duration. perDuration Repetition is noted by duration (see cycle note). semimonthly Twice a month (see cycle note)