The orange portion of the original formula increments the year when needed:
8 Months Later = If(Value(DateFormat(Date Purchased,"M"))=4, 12, Mod(Value(DateFormat(Date Purchased,"M"))+8,12)) & "/" & DateFormat(Date Purchased, "d") & "/" & Value(DateFormat(Date Purchased, "yyyy"))+If(DateFormat(Date Purchased,"M")>4, 1, 0)
If you're seeing incorrect results, perhaps this is an issue related to the locale setting (as with the 15 Days Later function in the post above).