日期计算  

CALL FUNCTION 'RP_CALC_DATE_IN_INTERVAL'
      EXPORTING
           date = l_date
           days = '00'
           months = p_month
           signum = '-'
           years = '00'
     IMPORTING
          calc_date = l_date.
date為輸入的日期，months要加或減的月份，也可以用days or years，signum為