Since writing this post, the code has changed dramatically. The code below still serves to describe the algorithm though. Read on!

After some hard searching online for help with automatically detecting user’s timezones with Javascript, I only came up with this one really good hit. Josh Fraser had done some preliminary work on the subject here. And his script worked really well for many cases.

However it soon became clear that there is an extreme abundance of ambiguities when it comes to time zones. And it all comes down to daylight saving time (DST), without which, the world would be a better place. :)

The difficult part is distinguishing between timezones,

• in the same UTC offset,
• in the same hemisphere,
• and that all use daylight saving time,
• but use daylight saving time differently!

My favorite example is the UTC+2 region, where many timezones differ only in which hour of the day DST starts yearly.

Anyway, for many reasons, it is very handy to have a robust determination of a user’s timezone at hand, I and wanted to do this the right way.

I set out trying to improve on Josh Fraser’s script and had two goals in mind:

• Automatically detect all unique modern time zones. (I don’t care about timezones as they were in the 18th century, and I don’t care that Helsinki time and Istanbul time are different historically, in modern time they are indistinguishable).
• Map these timezones to the Olson zoneinfo database, so that I can save a user’s time zone for use in system generated mails, batches, and even for displaying server generated pages to the user in their local time.

Using a simple algorithm wherein all known ambiguities regarding daylight savings are sorted out, I set about doing just this. The result of my work is now open sourced as the project jsTimezoneDetect on Bitbucket. And you can test the demo of it here.

Just to briefly describe the algorithm, it goes a little something like this. Imagine that you are in Alaska when you read this code:

function get_timezone_info() {
    // Alaska offset in january is -540 minutes from UTC
    var january_offset = get_january_offset(); 
 
    // Alaska offset in june is -480 minutes from UTC
    var june_offset = get_june_offset(); // returns 480
 
    // There's a -60 minutes offset difference between january and june in Alaska
    var diff = january_offset - june_offset; 
 
    // Diff = -60, so we know that this region uses daylight savings,
    // and is in the northern hemisphere. Also we know that the January
    // offset is the common denominator for the timezone, i.e 540 minutes = 9 hours
    if (diff < 0) {
        return {'utc_offset' : january_offset,
                'dst':  1,
                'hemisphere' : HEMISPHERE_NORTH}
    }
    // This is where we end up for southern hemisphere daylight savings time zones
    else if (diff > 0) {
        return {'utc_offset' : june_offset,
                'dst' : 1,
                'hemisphere' : HEMISPHERE_SOUTH}
    }
 
    // this is returned for non DST time zones
    return {'utc_offset' : january_offset,
            'dst': 0,
            'hemisphere' : HEMISPHERE_UNKNOWN}
}

Here are some return examples from the function, for different timezones:

// Central Australian Time (Audelaide):
{utc_offset=570, dst=1, hemisphere="south"}
 
// Central Australian Standard Time (Darwin)
{utc_offset=570, dst=0, hemisphere="N/A"}
 
// Central European Time
{utc_offset=60, dst=1, hemisphere="north"}
 
// Alaska
{utc_offset=-540, dst=1, hemisphere="north"}
 
// Mountain Time
{utc_offset=-420, dst=1, hemisphere="north"}

Using the info provided by this function I can map to a list of Olson timezone info keys. However, there are ambiguities. In many cases it is not enough to check January and June to make a granular enough distinction. Therefore I added the added knowledge of ambiguous time zones to the script, and when it detects possible ambiguities it will sort them out for you by checking known dates when DST started in different regions.

If you feel the need to use this script, check out the Bitbucket repository for the complete source code.

If you just want to test it, go to the demo page.