For instance, in the calendar of the Hadley Centre GCM, all months have 30 days. For example, the interval between 1 February 1996 and 1 March 1996 is one month, and equals 29 days in the standard calendar, but 30 days in the Hadley Centre model calendar, since 30 February is a valid date in the latter. Therefore the encoding of a time into an elapsed interval will depend on the calendar, and it is necessary to know the calendar when converting. The calendar attribute, described in the following sections, indicates the calendar in use. This standard permits two different methods, distinguished by their units , of encoding a time into a number.
Relative time is a more familiar method, but absolute time offers important advantages.