If you don't need the user's timezone immediately for their first request (which I assume is why you can't use a javascript solution), using getTimezoneOffset()

If you don't need the user's timezone immediately for their first request (which I assume is why you can't use a javascript solution), using getTimezoneOffset() works pretty well. Shameless plug: I wrote a gem to handle this in Rails ( https://github.com/zmillman/browser-timezone https://github.com/zmillman/browser-timezone )