The "Period" data type provides support for expressing anchored segments of the time axis, i.e. time intervals with a specific beginning and a specific end. A period is entered as a pair of dates, enclosed in square brackets ([]) and separated with a comma. The dates may be expressed in different granularities, but it is required that the starting date should be less or equal to the end date. The following examples illustrate acceptable values for a period slot:
- [2002, 2006]
- [2002-01, 2002-01]
- [2002-01-18 10:32:11, 2006-03-25 10:58:43]
- [2002-01-18 10:32:11, 2006-03]
Contrary, the following examples illustrate unacceptable values for a period slot:
- [2006-03-25, 2002-01-18] (ending date is before the starting date)
- [2006, 2006-10-01] (starting date represents the whole of the year 2006, which includes dates that are after the ending date).
Semantically, a period is considered to include both the starting and the ending time point, e.g. the period [2002-01, 2002-02-12] is considered to include the whole of January 2002 and the twelve first dates of February. This is particularly important for the notion of overlapping periods used in the context of non-multiple temporal types (discussed in the next paragraphs). According to the adopted semantics, periods [2002-01, 2002-02-12] and [2002-02-12, 2002-03] do overlap (since they both include February 12, 2002), while periods [2002-01, 2002-02-12] and [2002-02-13, 2002-03] do not overlap.
The special date values unknown and now may be used for the starting or the ending date of a period (or both). In such a case, no check is performed that the starting date should be before the ending date.