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.
