Timed addition to segments

Timed addition to segments

Hi there - hoping you can help me figure out a graceful way of doing this:

•      I want to add contacts to a Segment in Campaigns based on a Stage pick-list field value X in CRM - that bit's fine.
The problem is that I only want to add them to the Segment 90 DAYS after they are assigned stage X

•      In addition - if I get a solution to this - then I also need a way to 'backdate' this function to apply to all Contacts with stage field value X using the date of when they were marked X

A segment that is something like:
CONTACTS
with STAGE VALUE=X
with STAGE UPDATED TIME > 89 days


I hope I have explained it well enough.
Many thanks in advance,
David