There are two issues here. One is formatting a date - See Date Formatting and Calculating.
The other is getting this to happen when you tab out of a field. I have no idea how to do that. You can put a button somewhere that when clicked will fire an action that runs the Javascript to reformat the field. I would have to know more to come up with other solutions. Pls feel free to call me at 678-571-4179 in Atlanta, GA to discuss.
