Customer Exit Text Does Not Get Executed

Dear all,

I have a text variable to display the last date of a month, named ZT_LASTDATE.

Subsequently, I write an ABAP code in CMOD as follows:

CASE I_VNAM.

  WHEN 'ZT_LASTDATE'.
*    IF I_STEP = 2.
* Assign 0P_PER3 (Posting Period) value to v_month.
      CLEAR loc_var_range.
      READ TABLE i_t_var_range INTO loc_var_range WITH KEY VNAM = '0P_PER3'.
* Logic to convert Period 13, 14, 15 & 16 to Period 12.
      IF loc_var_range-low GT '012'.
        v_month ='12'.
* For normal periods (Period 01 - Period 12).
      ELSE.
        v_month = loc_var_range-low+1(2).
      ENDIF.

* Assign 0P_FYEAR (Fiscal Year) value to v_year.
      CLEAR loc_var_range.
      READ TABLE i_t_var_range INTO loc_var_range WITH KEY VNAM = '0P_FYEAR'.
      v_year = loc_var_range-low.

* Assign first date of the month i.e. 01 to v_date.
      v_date = '01'.

* Concatenate Year, Month & Date into ZW_DT1 as input
* for SLS_MISC_GET_LAST_DAY_OF_MONTH function module.
      CONCATENATE v_year v_month v_date INTO ZW_DT1.

* Call function module to get last date of the month.
* Input  : ZW_DT1 e.g. 20111201
* Output : ZW_DT2 e.g. 20111231
      CALL FUNCTION 'SLS_MISC_GET_LAST_DAY_OF_MONTH'
        EXPORTING
          day_in            = ZW_DT1
        IMPORTING
          last_day_of_month = ZW_DT2.

* Prepare last date of the month in DD.MM.YYYY format.
* ZW_DT2 date format        = 20111231
* l_s_range-low date format = 31.12.2011
      CLEAR l_s_range.
      l_s_range-low+0(2) = ZW_DT2+6(2).
      l_s_range-low+2(1) = '.'.
      l_s_range-low+3(2) = ZW_DT2+4(2).
      l_s_range-low+5(1) ='.'.
      l_s_range-low+6(4) = ZW_DT2+0(4).
      l_s_range-sign     = 'I'.
      l_s_range-opt      = 'EQ'.

* Send last date of the month output to ZT_LASTDATE variable.
      APPEND l_s_range TO e_t_range.
*    ENDIF.
ENDCASE.

However, when I execute my query, the variable text is displayed as &ZT_LASTDATE& and not as expected e.g. 31.12.2010.

I tried to debug it and found out that after the program stop at

WHEN 'ZT_LASTDATE'.

it went straight to ENDFUNCTION of INCLUDE ZXRSRU01.

May I know what went wrong here?

Thanks!