duration is a datatype that represents durations of time. The concept of duration being captured is drawn from those of [ISO 8601], specifically durations without fixed endpoints. For example, "15 days" (whose most common lexical representation in duration is "'P15D'") is a duration value; "15 days beginning 12 July 1995" and "15 days ending 12 July 1995" are not duration values. duration can provide addition and subtraction operations between duration values and between duration/dateTime value pairs, and can be the result of subtracting dateTime values. However, only addition to dateTime is required for XML Schema processing and is defined in the function ·dateTimePlusDuration·.