Actually there are a lot of FM to do this also. Check some below:
DAYS_BETWEEN_TWO_DATES
LEAP_DAYS_BETWEEN_TWO_DATES
HR_SEN_CALE_DAYS_DATE
ISB_DAYS_BETWEEN_TWO_DATES
FIMA_DAYS_BETWEEN_TWO_DATES
FIMA_DAYS_BETWEEN_TWO_DATES_2
FIMA_LEAP_DAYS_BETWEEN_2_DATES
SD_DATETIME_DIFFERENCE
Reference Thread: FM to Calculate difference in days between to dates