Bex Customer exit variable URGENT !!!

hello everyone,

i have to create a customer exit variable in BEx to fulfill the following scenario.

the user enters a value : mm/yyyy format. from this value we have to derive the current month / last year value , YTD value for this year and previous year. But i dont know how to write the code in ABAP in the exit. Can anyone help me out with that. Also is there any documentation where i can find out what the code means?

thanks

Laura.