Through Planet PHP I found an article on Pre-populating forms with the timezone . I'd normally add a comment instead, but the comment would almost be larger then the original post, so I am instead writing up an entry myself. The post describes several ways to obtain the user's timezone and use that to pre-fill a form. None of them are working properly though. I'll try to explain for each of them why not, but first of all it is important to know what a timezone actually is.
A timezone is a set of rules that determines the UTC offset for different times around the year for a specific location. Because of daylight savings time, a specific location can have two (or more) different UTC offsets depending on what point in time you look at it. Some areas might have had different rules in the past, even in the same country. An example here is China, where currently there is only one timezone, but previously there were multiple. Thus there are different timezones for the different locations in China, even though the current UTC offset is the same for all of them. Timezones are identified by Country/City or Country/Subcountry/City.
The offering by MaxMind allows you to link a country/region combination to Timezone identifier. For the US it subdivides this per state even. However, it misses the different timezones for Russia, Australia and even Indiana, USA.
IP2Location only provides a single UTC offset per IP range, totally ignoring daylight savings time.
The Hostip.info solution I can't access because phpclasses requires some stupid registration.
This is still not perfect of course, because it would for example give the same result for most of Europe (Europe/Berlin).
Figuring out the user's timezone can be done by using a high-accuracy database of IPs to lat/longitudes such as hostip.info and MaxMind offer, as well as a mapping from location to timezone. The latter however, is not available as far as I know. If however a data file that has proper definitions of the different timezone boundaries exist (mostly, on a per-province level would be enough), then such a tool can be easily build. I saw that OpenStreetMap has such a map, but I can't really find the raw data for that unfortunately. It would however, be awesome to have such a data file.