Functions on periods returning a period

[Seddryck]

• overlap: returns a new period beginning at the same instant than the period beginning the latest and ending at the same instant than the period beginning the soonest
• left-diff(period) : Returns the portion of the argument period that exists before the beginning of the parameter period when the periods overlap. When the periods overlap but there is no portion of the argument period before the beginning of the parameter period or the periods do not overlap, the function returns null. If either period is null, it returns null.
• right-diff(period) : Returns the portion of the argument period that exists from the end of the parameter period when the periods overlap. When the periods overlap but there is no portion of the argument period the argument period from the end of the parameter expression or if the periods do not overlap, it returns null. If either period is null, it returns null.

timeUnit is a string within the following potential values: year, quarter, month, week, day, hour, minute, second