How to capture user input for customer exit processing?

I need to calculate the number of working days elapsed in the current fiscal quarter BASED on the USER INPUT on the reporting front. i.e., say the fiscal quarter started on 1 July 2005 and if the user enters 10 July 2005, I should get the value 8 (Assume that Monday through Friday are all workdays). If the user enters 12 July 2005, I should get 10. I have written customer exits and know how to use factory calendar, but <b>THE CHALLENGE</b> is how do I <b>CAPTURE</b> the user input and use it in my exit? During the varible definition, if I select the check box "Ready for input" then the customer exit is not being processed and unless I check that box I can't get a user entry! If I look at the import values in the customer exit, I see i_t_var_range with type rrs0_t_var_range. My strong feeling is that this parameter gets the user input, but I am unable to use it as the customer exit is not being called if I make the user to input the data. Based on the empirical evidence, I felt that user input and customer exit can not co-exist!! Please somebody prove me wrong and let me know how can I use the user input to process my "customer-exit" variable. I would really appreciate any input from the BW community here.