To get "No Of days in each Month" based on Cal year/month

Hi BI gurus,

I am creating a query, which I need to create a Variable Formula (Cust Exit) for getting Number of Days in each month based on User Manual Input (Cal/Month).

My current coding:-

*TO calculate No Of Days in each Month
 
       WHEN 'ZVAR_NO_DAYS_IN_EACH_MONTH'.
         LOOP AT i_t_var_range INTO loc_var_range
              WHERE vnam = '0PCALMON'.
           CLEAR l_s_range.
           zcalyear = loc_var_range-low(4).
           zcalmonth2 = loc_var_range-low+4(2).
 
           CONCATENATE zcalyear zcalmonth2 '01' INTO zcalday.
 
           IF zcalmonth2 BETWEEN 01 AND 03.
             zcalyear = zcalyear - 1.
           ENDIF.
 
           CALL FUNCTION 'SLS_MISC_GET_LAST_DAY_OF_MONTH'
             EXPORTING
               day_in = zcalday
             IMPORTING
               last_day_of_month = zcalday1
             EXCEPTIONS
               day_in_not_valid  = 1.
 
           CONCATENATE zcalyear zcalmonth2 '01' INTO zcalday2.
           zdays = zcalday1 - zcalday2 + 1.
           l_s_range-low = zdays.
           l_s_range-sign = 'I'.
           l_s_range-opt =  'EQ'.
           APPEND l_s_range TO e_t_range.
         ENDLOOP.
         CLEAR: zcalyear, zcalmonth, zcalmonth2, zdays.

Although the Codes seems fine, when I executed the Query, it actually prompt-up an extra Manual Input fields.

The Calendar Year/Month(*) is OPCALMON, which is correct, but

Another Field prompted for No Of Days in Each Month.

I have double checked and The Formula Variables I created is Cust Exit, not manually entered.

Can anyone point out anything I missed in the code please?

Many thanks,
Vince