Auto-populate selection screen field

I have 2 selection-screen fields - quarter and year.

If I select quater as Q2 and year as 2011, my 3rd selecion screen field should auto-populate with start and end date of that year quarter.

I am able to achieve this once I press ENTER, after populating year. But on manullay filling quarter and year and clicking the third field using cursor, it does not auto-populate.

I want the dates to auto-populate as soon as the cursor moves from year - by doing tab or mouse click or enter hit.