That isn't available on the http-request, but if javascript is enabled, it is available there via Date.getTimezoneOffset()
. You may also want to compare the time in javascript to the server-time (which you could send down in the response as milliseconds since Jan 1 1970) to get a more accurate delta to the server.
You can of course use javascript to populate a hidden form-field so your server can know the time, but that excludes the first request. IMO, it is simpler to just handle the time offsets at the UI.
Note also that you can't do much if javascript is disabled (except maybe a drop-down of possible offsets to choose from).