Help with Customer exit variable

Hi Guys,

I need your urgent help. I have a requirement where I need to create a variable (on CALDAY) in a report which should use the input value given for another variable (created on CALMONTH). To put in words more clearly, CALMONTH is a variable that is mandatory for input but the CALDAY variable should not appear on the selection screen. I was told that I should write a customer exit to make this happen. So can someone please explain me the process of creating a customer exit and give me the code.

WHEN CALMONTH values are selected to be 07/2006 - 08/2006... CALDAY values should be 07/01/2006 to 08/31/2006.

CALMONTH is user entry, mandatory, range and multiple selection variable.

Early reply is highly appreciated and duely rewarded. Please let me know if I am not clear about the requirement. Thanks a bunch in advance.