Mapping US zip code to time zone [closed]

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 (again, there are exceptions). Depending on your needs, 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.

If you want, you can also get a feel for the timezone by asking the browser Josh Fraser has a nice write up on it here

var rightNow = new Date();
var jan1 = new Date(rightNow.getFullYear(), 0, 1, 0, 0, 0, 0);
var temp = jan1.toGMTString();
var jan2 = new Date(temp.substring(0, temp.lastIndexOf(" ")-1));
var std_time_offset = (jan1 - jan2) / (1000 * 60 * 60);

The second thing that you need to know is whether the location observes daylight savings time (DST) or not. Since DST is always observed during the summer, we can compare the time offset between two dates in January, to the time offset between two dates in June. If the offsets are different, then we know that the location observes DST. If the offsets are the same, then we know that the location DOES NOT observe DST.

var june1 = new Date(rightNow.getFullYear(), 6, 1, 0, 0, 0, 0);
temp = june1.toGMTString();
var june2 = new Date(temp.substring(0, temp.lastIndexOf(" ")-1));
var daylight_time_offset = (june1 - june2) / (1000 * 60 * 60);
var dst;
if (std_time_offset == daylight_time_offset) {
    dst = "0"; // daylight savings time is NOT observed
} else {
    dst = "1"; // daylight savings time is observed
}

All credit for this goes to Josh Fraser.

This might help you with customers outside the US, and it might complement your zip approach.

Here is a SO questions that touch on getting the timezone from javascript