Mapping US zip code to time zone
When users register with our app, we are able to infer their zip code when we validate them against a national database. What would be the best way to determine a good potential guess of their time zone from this zip code?
We are trying to minimize the amount of data we explicitly have to ask them for. They will be able to manually set the time zone later if our best guess is wrong. I realize zip codes won't help with figuring out the time zone outside the US, but in that case we'd have to manually ask anyway, and we deal predominantly with the US regardless.
I've found a lot of zip code databases, and so far only a few contain time zone information, but those that do are not free, such as this one. If it's absolutely necessary to pay a subscription to a service in order to do this, then it will not be worth it and we will just have to ask users explicitly.
Although language isn't particularly relevant as I can probably convert things however needed, we're using PHP and MySQL.
I just found a free zip database that includes time offset and participation in DST. I do like Erik J's answer, as it would help me choose the actual time zone as opposed to just the offset (because you never can be completely sure on the rules), but I think I might start with this, and have it try to find the best time zone match based on offset/dst configuration. I think I may try to set up a simple version of Development 4.0's answer to check against what I get from the zip info as a sanity test. It's definitely not as simple as I'd hope, but a combination should get me at least 90% sure of a user's time zone.
Most states are in exactly one time zone (though there are a few exceptions). Most zip codes do not cross state boundaries (though there are a few exceptions).
You could quickly come up with your own list of time zones per zip by combining those facts.
Here's a list of zip code ranges per state, and a list of states per time zone.
You can see the boundaries of zip codes and compare to the timezone map using this link, or Google Earth, to map zips to time zones for the states that are split by a time zone line.
The majority of non-US countries you are dealing with are probably in exactly one time zone. You may want to look at where your top-N non-US visitors come from and just lookup their time zone.
I created an Open-Source (MIT licensed) MySQL database table that cross-references zip codes and timezones and uploaded it to sourceforge.net:
http://sourceforge.net/projects/zip2timezone/files/
It is sourced from four locations (primary Yahoo PlaceFinder API - thanks @Chris N)
See the README file for more information and instructions.
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