If you know you're on a network -- and Dropbox does -- then I'd fetch the time from a time server somewhere, perhaps Dropbox itself, or time.nist.gov. Of course, then you're at the mercy of the user's hosts file, I suppose.

If the time is just used to check for certificate expiration, maybe nobody considers it worth worrying about. Which I guess is fine as far as it goes... but the other day I couldn't get Dropbox to do its thing when my clock was only a few hours off. It didn't seem likely that an SSL date check was involved in that case.

Well. Chrome and Firefox won't let you log in to GMail, for instance. IE at least allows you to continue if you really want.

It's a common thing on school/university computers, where for various reasons, system time is always wrong and it requires Administrator privilenges to change it (I can't understand why).