Try this:
years = YEAR(TODAY).
lastbirthday = DATE(MONTH(birthday), DAY(birthday), years) NO-ERROR.
IF ERROR-STATUS:ERROR THEN lastbirthday = DATE(2, 28, years).
IF lastbirthday > TODAY THEN DO:
lastbirthday = DATE(MONTH(birthday), DAY(birthday), years - 1) NO-ERROR.
IF...