populating a text variable via a customer exit

I need to populate a text variable with a date range which is based off a date that is entered by the user. the user enters the Quarter, like 4/2006 and the report shows a rolling 4 quarters. I need to display this range to the user in the title. so they would want to see 01/2006 through 04/2006. I tired to do this but it only allows for 01/2006 to be entered in the field. How do i get the whole thing? do i need to create 2 separate variables?