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This is only an aproximation of duration, more work still to do. See the schema url for more info on duration.
http://www.w3.org/TR/xmlschema-2/
[Definition:] duration represents a duration of time. The value space of duration is a six-dimensional space where the coordinates designate the Gregorian year, month, day, hour, minute, and second components defined in 5.5.3.2 of [ISO 8601], respectively. These components are ordered in their significance by their order of appearance i.e. as year, month, day, hour, minute, and second.
3.2.6.1 Lexical representation The lexical representation for duration is the [ISO 8601] extended format PnYn MnDTnH nMnS, where nY represents the number of years, nM the number of months, nD the number of days, 'T' is the date/time separator, nH the number of hours, nM the number of minutes and nS the number of seconds. The number of seconds can include decimal digits to arbitrary precision.
The values of the Year, Month, Day, Hour and Minutes components are not restricted but allow an arbitrary integer. Similarly, the value of the Seconds component allows an arbitrary decimal. Thus, the lexical representation of duration does not follow the alternative format of 5.5.3.2.1 of [ISO 8601].
An optional preceding minus sign ('-') is allowed, to indicate a negative duration. If the sign is omitted a positive duration is indicated. See also ISO 8601 Date and Time Formats (D).
For example, to indicate a duration of 1 year, 2 months, 3 days, 10 hours, and 30 minutes, one would write: P1Y2M3DT10H30M. One could also indicate a duration of minus 120 days as: -P120D.
Reduced precision and truncated representations of this format are allowed provided they conform to the following:
If the number of years, months, days, hours, minutes, or seconds in any expression equals zero, the number and its corresponding designator *may* be omitted. However, at least one number and its designator *must* be present. The seconds part *may* have a decimal fraction. The designator 'T' shall be absent if all of the time items are absent. The designator 'P' must always be present. For example, P1347Y, P1347M and P1Y2MT2H are all allowed; P0Y1347M and P0Y1347M0D are allowed. P-1347M is not allowed although -P1347M is allowed. P1Y2MT is not allowed.
