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Handling timezone conversion with PHP DateTime

The target audience for our server monitoring product, Server Density, is technical users – sysadmins and developers. As such, each weekend we are going to publish a technical article focused on a technology we use or a programming problem we have solved.

Last night, we pushed out a rewrite of the timezone handling in SD. This now allows you to select your location (or nearest major city within your timezone) so the dates/times displayed in the interface can be automatically converted to your timezone, including handling of DST. This is accomplished using the new DateTime class introduced into PHP 5.2. Prior to the release of 5.2, conversion into different timezones required either use of the Pear Date package or manually calculating offsets.

The first complexity that you need to deal with as a programmer is how the timezones are calculated relative to the server’s default timezone setting. Since PHP 5.1, all the date/time functions create times in the timezone of the server. The timezone can be set programmatically as of PHP 5.2 using the date_default_timezone_set() function. So if you call the date() function without specifying a timestamp as the second parameter and the timezone is set to GMT, then the date will be in the +0000 timezone; equally if you set the timezone to be New York and it is winter, the timezone will be -0500 (-0400 in summer).

This means that if you need the date in GMT then you need to know what the offset is of the date you’re working with so that if necessary, you can do the calculation to convert it to +0000. When would you need to do this? Well, the MySQL TIMESTAMP field type stores the timestamp internally using GMT (UTC) but always returns it in the server’s timezone. This means if you want to do any SELECT statements, you always need to convert the timestamp you pass in your SQL to UTC.

This sounds quite complex but in reality, you can use the DateTime class to do most of the hard work. You first need to get the user to specify their timezone. This will then be attached to any DateTime object you create so that the right offset can be calculated based on it. The PHP manual provides a list of all the acceptable timezone strings which can be used. There is also a PHP function which will output the list of timezones. Server Density uses this to generate a list of timezones as a drop menu for the user to select from.

Once you have the user’s timezone stored, you can create a DateTimeZone object from it. This will be used to do all the offset calculations.

$userTimezone = new DateTimeZone($userSubmittedTimezoneString);

Now to convert a date/time into the user’s timezone, you simply need to create it as a DateTime object:

$myDateTime = new DateTime('2009-03-21 13:14');

This will create a DateTime object which has the time specified. The parameter accepts any format supported by strtotime() and if you leave it empty, it will default to “now”.

It is important to note that the time created will be in the timezone that has been set as the default for the server. This is relevant because the offset calculated will be relative to that timezone. For example, if the server is GMT and you want to convert to Paris time, it will require adding 1 hour. However, if the server is in the Paris timezone then the offset will be zero. You can force the timezone that you want $myDateTime to be in by specifying the second parameter as a DateTimeZone object. So for example, if you want it to be 13:14 on 21st March 2009 in GMT, then you would need to use this code:

$gmtTimezone = new DateTimeZone('GMT');
$myDateTime = new DateTime('2009-03-21 13:14', $gmtTimezone);

To double check, you can do:

echo $myDateTime->format('r');

which would output Sat, 21 Mar 2009 13:14:00 +0000.

The final step is to work out the offset from your DateTime object to the user’s timezone so you can do the appropriate calculation to convert it into that timezone. This is where the $userTimezone DateTimeZone object comes in because you use the getOffset() method:

$offset = $userTimezone->getOffset($myDateTime);

This will return the number of seconds that you need to add to $myDateTime to convert it into the user’s timezone. Therefore:

$userTimezone = new DateTimeZone('America/New_York');
$gmtTimezone = new DateTimeZone('GMT');
$myDateTime = new DateTime('2009-03-21 13:14', $gmtTimezone);
$offset = $userTimezone->getOffset($myDateTime);
echo $offset;

will print -14400, or 4 hours (because New York is on DST).

This is where the DateTime::add method would come in handy because we could just add the offset. Unfortunately, it is not available until PHP 5.3 so we need to do the calculation manually by converting to Unix timestamp.

echo date('Y-m-d H:i', $myDateTime->format('U') + $offset)

would output 2009-03-21 09:14 which is the correct conversion from 2009-03-21 13:14 London GMT to New York time.

  • http://www.nihongonotes.com Chris

    Thanks for this, helped me find the answer I was looking for.

    Instead of calculating the offset I use setTimezone by replacing:
    $offset = $userTimezone->getOffset($myDateTime);
    echo date(‘Y-m-d H:i’, $myDateTime->format(‘U’) + $offset);

    with:
    $myDateTime->setTimezone($userTimezone);
    echo $myDateTime->format('Y-m-d H:i');

    This is working in PHP 5.2.8

  • http://blog.getgoaling.com Sandeep

    Have to implement timezone functionality on my site. The article provides a good starting point. Thanks.

  • Blake

    We use the same method as Chris suggested (i.e. use setTimezone to switch between timezones). Using the setTimezone method seems cleaner than calculating the offset and adding it to the date.

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  • Josh

    You can shorten the code even more by doing this (converts GMT to New York):

    $myDateTime = new DateTime('2009-03-21 13:14', new DateTimeZone('GMT'));
    $myDateTime->setTimezone(new DateTimeZone('America/New_York'));
    echo $myDateTime->format('Y-m-d H:i');

    • http://www.serverdensity.com David Mytton

      This doesn’t do any conversion, it just changes the timezone assigned to the DateTime object. It’s what we tried first and it doesn’t work.

      • Josh

        I’m not sure what you are talking about, but it appears to work fine with PHP 5.2.11:

        $myDateTime = new DateTime(’2009-03-21 13:14′, new DateTimeZone(‘GMT’));
        echo $myDateTime->format(‘r’);

        outputs Sat, 21 Mar 2009 13:14:00 +0000

        $myDateTime->setTimezone(new DateTimeZone(‘America/New_York’));
        echo $myDateTime->format(‘r’);

        outputs Sat, 21 Mar 2009 09:14:00 -0400

      • Sam

        Oddly enough, I couldn’t get the original code to work while converting from an Eastern start time to anything else. The one Josh posted works perfectly. Thanks to both for providing a straightforward solution.

      • Michael

        Hey, just thought I would add that according to the docs setTimezone should indeed cause a conversion to occur. Tthe examples shown appear to indicate this (even though the textual description does not explicit state it):


        $date = new DateTime('2000-01-01', new DateTimeZone('Pacific/Nauru'));
        echo $date->format('Y-m-d H:i:sP') . "n";

        $date->setTimezone(new DateTimeZone('Pacific/Chatham'));
        echo $date->format('Y-m-d H:i:sP') . "n";

        Output:


        2000-01-01 00:00:00+12:00
        2000-01-01 01:45:00+13:45

    • http://richardwillia.ms Richard Williams

      This worked fine for me.

      Thanks for a good tutorial, pointed me in the right direction after going mad with bugs in PEAR Date.

      I’ve written up a simple function based on Josh’s script…

      http://richardwillia.ms/blog/2011/04/time-zone-conversion-using-datetime-class/

  • I. Zuiff

    Are you having a problem using
    date_default_timezone_set() ?

    It seems to be working for me. My server is in Arizona and I am on the east-coast.

    output: Wed,March 24, 2010, 12:27:17 PM EDT

  • I. Zuiff

    here’s the code:

    Are you having a problem using
    date_default_timezone_set() ?

    It seems to be working for me. My server is in Arizona and I am on the east-coast.

    date_default_timezone_set(‘America/New_York’);
    echo date(‘D,F j, Y, h:i:s A T’);

    output: Wed,March 24, 2010, 12:27:17 PM EDT

  • Chris

    This helped me a lot, thanks!

  • http://n/a Tommy

    I face a similar challenge. I need to convert a user’s chosen time to my server’s time. e.g. I have a form which a user can select a date, time, and their timezone. Upon submission of the form, I want to timestamp the file with my timezone’s time. My server is in KS, say a user in CA chooses July 1, 2010 8:00 AM Pacific timezone…I want my server timestamp the file July 1, 2010 10:00 AM Central time. Help?

  • WayFarer

    And timezone conversion from server’s timezone to user’s in one line (convinuent for templates):

    echo date_create($userDate)->setTimezone(new DateTimeZone($userTimezone))->format(‘Y-m-d H:i’);

    or from one timezone to another:
    echo date_create($userDate, new DateTimeZone($userTimezoneFrom))->setTimezone(new DateTimeZone($userTimezoneTo))->format(‘Y-m-d H:i’);

  • Pingback: Automatic timezone conversion with JavaScript « Boxed Ice Blog

  • http://www.gmtslider.com GMTSlider.com

    That’s exactly the code i was looking for so long! Thanks!

  • Skibbler Elf

    This post has helped a ton. It does help to know your user’s timezone…then you can figure the offset and store the correct time (most likely UTC) in your dB.

    Ciao

  • http://elastik.sourceforge.net/ James

    Very useful, thanks, just put Timezone handling into our open source project :-)

  • Jeff

    I see a lot of people have already commented more or less what I was going to say — it’s much easier to use the setTimezone() method. However I’d like to point out that it’s not only easier, it’s really the way you have to do it (if you want it to work). First off, the approach your suggesting only appears to work because you happen to be calculating your offset against a DateTime object which happens to be using the GMT timezone. getOffset() always returns the offset from GMT, regardless of the timezone set on the DateTime object you use. In fact, the only reason you need the DateTime object is because that world time is used to determine what the offset is — The offset changes for TimeZones which follow daylight savings time and depending on what time of the year it is, the offset will change.
    Mostly I want to point out though that your method of changing the time using the offset will leave you with a new DateTime object that has the wrong time. Or rather it will have the right time, but the wrong TimeZone. Worse it won’t necessarily be the right time — if you happen to offset the time across a clock change (spring forward, fall back) you’ll end up with the wrong time and the wrong timezone.
    Here’s what I’m getting at. Suppose the following time
    date_create(’2011-03-13T01:30′, timezone_open(‘America/Chicago’));
    That’s just 30 minutes before the clocks get turned forward. If you add an hour with the following line
    $sometime->add(date_interval_create_from_date_string(’1 hour’));
    The resulting time is actually 3:30am because the hour from 2-3am doesn’t exist. Also, the clocks in the US all change at 2am based on local time, so strangely enough, in the fall you end up with a situation where 2:30am in Chicago is actually the second 2:30am in New York.
    If you use the setTimezone method though you’ll find that your times always change consistently because under the covers they’re always tracking UTC (GMT).

  • http://www.arpatech.co arpatech

    works for me.

    setTimezone(new DateTimeZone(‘Asia/Manila’));
    echo $myDateTime->format(‘Y-m-d H:i’);
    ?>

  • Gary

    Thanks for this. I found the information and technique very useful.

  • Vibha

    Hi,

    Thanx for the script,

    I am showing server time and client time both on a page, used different variables. Script works fine in Firefox and Chrome, but in Safari, IE9 and Opera, it is not working, either or both times are not showing.
    Please help

  • Nick

    Thanks for this

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