Relative Dates in Xpath Constraint

I have an entity which contains appointments.  Each appointment has a start date/time.  I would like to retrieve a grid that shows all appointments whose start date/time is greater than 3 weeks prior to the current date.  I have tried a number of different approaches, but haven’t found a way to do this.  I know about the tokens ('[%BeginOfCurrentMonth%]', '[%BeginOfCurrentYear%]', etc.), but would like a relative date (i.e. 3 weeks before current date/time, 6 weeks before current date/time, etc.) Any ideas welcome!
1 answers

To accomplish this you can use the following construct:

[DateAttribute > '[%BeginOfCurrentDay%] - 21 * [%DayLength%]']