Ah ok, I didn't think of that. Perhaps if you created a measure group from your candidate data you could include a StartTermDateKey (or however you would name it) in the source which could then be used in calculations if you linked it to the Date dimension in the cube in a role playing context (possibly hidden if you're only using it for calculation).

Role Playing Dimensions:

http://technet.microsoft.com/en-us/library/ms174487%28v=sql.90%29.aspx