If I remember correctly, you can use complex xpath tokens for this, for example:
[mydateattribute < '[%CurrentDateTime%] - 3 * [%WeekLength%]']
Note the position of the quotes
You can do some simple math with the CurrentDateTime:
[LastOnline > '[%CurrentDateTime%] - 5 * [%MinuteLength%]']
The xpath auto-complete doesn't suggest the length options but they do work. HourLength, DayLength, WeekLength etc.
Just make sure you keep it all in the quotes.