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 (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

Google has a specific API for this: https://developers.google.com/maps/documentation/timezone/

eg: https://maps.googleapis.com/maps/api/timezone/json?location=40.704822,-74.0137431&timestamp=0

{
  dstOffset: 0,
  rawOffset: -18000,
  status: "OK",
  timeZoneId: "America/New_York",
  timeZoneName: "Eastern Standard Time"
}

They require a unix timestamp on the querystring. From the response returned it appears that timeZoneName takes into account daylight savings, based on the timestamp, while timeZoneId is a DST-independent name for the timezone.

For my use, in Python, I am just passing timestamp=0 and using the timeZoneId value to get a tz_info object from pytz, I can then use that to localize any particular datetime in my own code.

I believe for PHP similarly you can find "America/New_York" in http://pecl.php.net/package/timezonedb

Ruby gem to convert zip code to timezone: https://github.com/Katlean/TZip (forked from https://github.com/farski/TZip).

> ActiveSupport::TimeZone.find_by_zipcode('90029')
 => "Pacific Time (US & Canada)"

It's fast, small, and has no external dependencies, but keep in mind that zip codes just don't map perfectly to timezones.

This will download a yaml file that will map all timezones to an array of their zipcodes:

curl https://gist.githubusercontent.com/anonymous/01bf19b21da3424f6418/raw/0d69a384f55c6f68244ddaa07e0c2272b44cb1de/timezones_to_zipcodes.yml > timezones_to_zipcodes.yml

e.g.

"Eastern Time (US & Canada)" => ["00100", "00101", "00102", "00103", "00104", ...]
"Central Time (US & Canada)" => ["35000", "35001", "35002", "35003", "35004", ...]
etc...

If you prefer the shortened timezones, you can run this one:

curl https://gist.githubusercontent.com/anonymous/4e04970131ca82945080/raw/e85876daf39a823e54d17a79258b170d0a33dac0/timezones_to_zipcodes_short.yml > timezones_to_zipcodes.yml

e.g.

"EDT" => ["00100", "00101", "00102", "00103", "00104", ...]
"CDT" => ["35000", "35001", "35002", "35003", "35004", ...]
etc...

See these links for the Olson database:

• http://en.wikipedia.org/wiki/Tz_database
• ftp://ftp.iana.org/tz/tz-link.html (yes, ftp) http://www.twinsun.com/tz/tz-link.htm
• ftp://ftp.iana.org/tz/releases ftp://elsie.nci.nih.gov/pub/
• https://iana.org/time-zones

I've been working on this problem for my own site and feel like I've come up with a pretty good solution

1. Assign time zones to all states with only one timezone (most states)

and then either

2a) use the js solution (<https://stackoverflow.com/questions/2319451/javascript-php-and-timezones>;) for the remaining states

or

2b) use a database like the one linked above by @Doug.

This way, you can find tz cheaply (and highly accurately!) for the majority of your users and then use one of the other, more expensive methods to get it for the rest of the states.

Not super elegant, but seemed better to me than using js or a database for each and every user.

In addition to Doug Kavendek answer. One could use the following approach to get closer to tz_database.

1. Download [Free Zip Code Latitude and Longitude Database]
2. Download [A shapefile of the TZ timezones of the world]
3. Use any free library for shapefile querying (e.g. .NET Easy GIS .NET, LGPL).

var shapeFile = new ShapeFile(shapeFilePath);
var shapeIndex = shapeFile.GetShapeIndexContainingPoint(new PointD(long, lat), 0D);
var attrValues = shapeFile.GetAttributeFieldValues(shapeIndex);
var timeZoneId = attrValues[0];

P.S. Can't insert all the links :( So please use search.

Also note, that if you happen to be using Yahoo geocoding service you can have timezone information returned to you by setting the correct flag.

http://developer.yahoo.com/geo/placefinder/guide/requests.html#flags-parameter

There's actually a great Google API for this. It takes in a location and returns the timezone for that location. Should be simple enough to create a bash or python script to get the results for each address in a CSV file or database then save the timezone information.

https://developers.google.com/maps/documentation/timezone/start

Request Endpoint:

https://maps.googleapis.com/maps/api/timezone/json?location=38.908133,-77.047119&timestamp=1458000000&key=YOUR_API_KEY

Response:

{
   "dstOffset" : 3600,
   "rawOffset" : -18000,
   "status" : "OK",
   "timeZoneId" : "America/New_York",
   "timeZoneName" : "Eastern Daylight Time"
}