Oh, I see the difference. In my test program I defined S_BUDAT as seen below.
TABLES: vbak.
SELECTION-SCREEN BEGIN OF SCREEN 400 AS SUBSCREEN.
SELECT-OPTIONS: s_datum FOR vbak-erdat.
SELECTION-SCREEN END OF SCREEN 400 .
Oh, I see the difference. In my test program I defined S_BUDAT as seen below.
TABLES: vbak.
SELECTION-SCREEN BEGIN OF SCREEN 400 AS SUBSCREEN.
SELECT-OPTIONS: s_datum FOR vbak-erdat.
SELECTION-SCREEN END OF SCREEN 400 .