How to add to a date but not add time?

How to add to a date but not add time?

I have a field called Checkout_date that has a date without showing a time (i.e. 1-01-2011)

I have a Due_Date field that has the following formula Checkout_date.addMonth(4)

The result that gives me is 5-01-2011 02:00:00

How can I make it so it doesn't show or doesn't ever include the time, only the date?