I don't think it's trivially easy. Most signups that require a correct local time solicit the user for their timezone. It should be possible though... as there are certainly auto-detect-location websites out there. If you had location, you could cross reference it with a location-to-zip-code kind of thing and then a zip-code-to-time-zone kind of thing. Makes me tired thinking about it though, you might wanna just add a field and ask.