In XPath it is possible to form so-called token expressions to add to or subtract from the current date. These expressions can be compared to an attribute of type DateTime.
Examples:
[Date >= '[%CurrentDateTime%] - 30 * [%DayLength%]']
[Date <= '[%BeginOfCurrentDay%] + 2 * [%WeekLength%]']
The topmost example seems to be the one you're after.