Automatic time format in form field?

Hello again!

How can I have a form field automatically fill some aspects of the data that's input? I've seen it before, like a phone# field automatically including the dashes.

I would like to automatically include colons in a time field. So if I were to type "081500" (or even "81500"), it would complete it as "08:15:00".

Any ideas? Thanks!
Willys