" This FUNCTION gives you the last day OF the month

DATA(lv_doc_date) = SY-datum.

CALL FUNCTION 'RP_LAST_DAY_OF_MONTHS'
EXPORTING
day_in            = lv_doc_date
IMPORTING
last_day_of_month = lv_doc_date
EXCEPTIONS
day_in_no_date    = 1
others            = 2
.
IF SY-SUBRC <> 0.
  *     MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO
  *       WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.
ENDIF.