It would depend on what date you're looking at (e.g. Actual Completion Date, Planned Completion Date, etc.) but here's an example of filtering tasks where the Actual Completion Date is >= 12 months ago:
Actual Completion Date gte
If you were looking at Text Mode, the code would look like this:
actualCompletionDate=$$TODAY-12m
actualCompletionDate_Mod=gte