These sorts of things can be done with the lead time functionality. For example:
A comment on the OP's interchangeable use of "2 weeks" and "14 days" -- you may not get the results you expect unless you have set your conversion factors accordingly; they default to 1 week being 5 work days, not 7. See this thread for more details:
http://forums.omnigroup.com/showthread.php?t=10949