Edit: I have some updates to this formula that I’ve explained in the post below.
You’re requirement to back Feb 29th by a day if it’s not a leap year does complicate it a bit, but that’s not a problem. It just makes for a formula that’s much larger.
Here is the formula if we were to allow Feb 29th to roll over to Mar 1st.
((Now-DAY(Now)+15)+(Years*365.25))
-
DAY((Now-DAY(Now)+15)+(Years*365.25))
+
DAY(Now)
To get the leap year date of Feb 29th to back date to Feb 28th instead of Mar 1st, then we need to duplicate a lot of the formula and calculate the month offset to subtract the number of months different as days. So the difference of the current month of February (2), is subtracted from the calculated date of March (3), and the difference (1) is subtracted as a day which rolls March 1st to February 28th.
((Now-DAY(Now)+15)+(Years*365.25))
-
DAY((Now-DAY(Now)+15)+(Years*365.25))
+
DAY(Now)
-
(MONTH(((Now-DAY(Now)+15)+(Years*365.25))
-
DAY((Now-DAY(Now)+15)+(Years*365.25))
+
DAY(Now))-MONTH(NOW))
I feel pretty comfortable that this will work with virtually an unlimited number of years since we are still accounting for leap years by multiplying by 365.25 instead of just 365.