In reply to Lex.

I agree, Lex, in this case (on multiples of 4 years after a Feb 29), YRDIF incorrectly increments the age on the 28th of the month instead of the 29th. No one born February 29th is going to celebrate their fortieth birthday on the 28th when there is a 29th in that year! The following statement, using the new-ish CONTINUOUS method for INTCK, calculates it correctly (as an integer) in all cases:

age3 = INTCK('YEAR', BirthDate, EndDate, 'C');