well, for almost everyone this information is contained within the IP anyway, though.

In what sense is the user's local time zone "contained within" the IP? The only way I know to get from an IP address (i.e. those four eight-bit integers separated by period signs) to a client-side timezone is first to use a Geo IP lookup table to obtain a physical location (usually, but not always correct), and then use a timezone database to look up the current political timezone in that location. Sure, some server setups will automate this for you so that the already-looked up information is contained within the request object that your chosen language/framework supplies. Is there something I've missed about those four eight-bit integers somehow directly encoding information that specifies the user's timezone, or did you mean something different?