Hacking Thy Fearful Symmetry

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Webpage Timezone Hopping

October 20, 2012
javascript

Timezones can be darn confusing. Personally, daylight saving is enough to throw me into bi-annual bouts of confusion. So when I deal with information on a web site posted by someone in a different timezone, talking about events happening in yet another timezone, it usually doesn't take very long before my head begins to swim. Of course, the typical way to get around those timezonish shenanigans is to display all times relative to UTC... which usally means that a fourth timezone is thrown in the mix. Madness.

So I thought, wouldn't be nice to be able to switch the times back and forth on a webpage, such that you don't have to juggle the time differences in your head, but rather just see the full thing first from your timezone perspective, then from the other guy's?

On Your Mark(up)

First thing that we want to do is to mark the times and dates on the page so that we'll be able to manipulate them.


<p>We shall meet at <span class="datetime" timezone="Etc/UTC">2012/10/20 12:34</span> at the
docks. We strike at <span class="datetime">2012/10/20 14:56</span>.</p>

And because the information is already there, let's add a css rule to display it:


<style>
    span.datetime:after {
        content: " (" attr(timezone) ")";
    } 
</style>

Get(), Set()

Now comes the hard part. We want to move times from one timezone to another. And it'd be cool to automatically recognize in which timezone the current user is, to provide a sensible default to our display. Trying to rewrite that from scratch would be silly, instead, let's use timezone-js for the timezone manipulations and jstimezonedetect for the detection. On, by the by, for the following code to work, I had to slightly tweak timezone-js. My version of the code is here.

With those two libraries as our base, we have left to do is to provide the glue:


function changeTimes() {
    var new_tz = $('select#timezone').val();

    $('.datetime').each(function(){
        var t = $(this);
        var tz = t.attr('timezone') || 'Etc/UTC';
        var dt = new timezoneJS.Date(t.text(), tz);
        dt.setTimezone(new_tz);
        t.attr('timezone',new_tz);
        t.text(dt.toString());
    });
}

$(function(){
    timezoneJS.timezone.zoneFileBasePath = '/time/tz';
    timezoneJS.timezone.init();

    $('select#timezone').val(jstz.determine().name()).change( changeTimes );

    changeTimes();
});

and provide the choice to the user.


<form>
    <select id="timezone" >
        <option selected="selected">Etc/UTC</option>
        <option>America/Montreal</option>
        <option>America/New_York</option>
        <option>Asia/Tokyo</option>
    </select>
</form>

Go!

Et voilà. A bare-bone example can be played with here. It's not the prettiest thing yet, but it's perfectly functional.

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About the author

Yanick Champoux
Perl necrohacker , ACP writer, orchid lover. Slightly bonker all around. he/him/his