Hi Shawn,
Your code is fine and it should work. I copied/executed your code and it gave the expected result.
OR you can alternatively use the FM : FIMA_DAYS_AND_MONTHS_AND_YEARS. But if you will check the coding of this FM , it also uses the same principle of subtracting the two dates.
Let me know if you still cant get the result.
Thanks
Deepak Sharma