Off-set Variable using Customer Exit

Hello Gurus,

I am trying to create a BEx report.I need to create 2 variables on 0calweek one to get current,the other to get 3 weeks back data and the last one to get 13 weeks back data.They will be off-set variable one -3 the other -13( By the way can some one please tell me how the 2 variables will work on 1 report).And need to have a Customer exit behind it.What the code is basically doing when its my vairable say ZCALWEEK it needs to go into a custom table ex: ZTABLE and for 0calday get the load type say in my case 7 and then the variable needs to read that date and convert it to 0CALWEEK.

My code in ZXRSRU01:

CASE i_vnam.

WHEN 'ZCALWEEK'.

IF i_step = '1'.

SELECT SINGLE low

INTO l_s_var_fr

FROM ztable

WHERE iobjnm = '0CALDAY' AND

load_Type= '7'.

l_s_range-low = l_s_var_fr.

l_s_range-high = l_s_var_fr.

l_s_range-sign = 'I'.

l_s_range-opt = 'EQ'.

APPEND l_s_range TO e_t_range.

ENDIF.

ENDCASE.

But when i run the result the popup comes and when i check it says :Value 09/0704 for variable Cal Week from ztable date is invalid.

Please help.

Thanks.