To GMT or not to GMT

A leap day with a post on Date/Time issues seems fitting...

Earlier today, on twitter, @skoop asked: "dear #lazyweb, when I use DateTimeZone('GMT'), why does format('e') output UTC?" What he means is that:

$date = new DateTime('now', new DateTimeZone('GMT'));
echo $date->format(DateTime::RFC2822 . ' e' );

which shows:

Wed, 29 Feb 2012 16:26:23 +0000 UTC

As you can see that has UTC and not GMT as you might expect.

If you look closely at the documentation for the "Other" group of timezones, it lists with the GMT timezone as warning: "Please do not use any of the timezones listed here (besides UTC), they only exist for backward compatible reasons." If you use GMT as timezone identifier in the constructor to DateTimeZone, PHP will instead use the correct UTC in output. When you create a DateTimeZone object like this, you will always get a "type 3" DateTimeZone object:

$date = new DateTime('now', new DateTimeZone('GMT'));
var_dump($date);

which shows:

object(DateTime)#1 (3) {
  ["date"]=>
  string(19) "2012-02-29 16:30:51"
  ["timezone_type"]=>
  int(3)
  ["timezone"]=>
  string(3) "UTC"
}

Now apparently some systems *cough*Silverlight*cough* require GMT to be used. GMT is not a timezone, but just a timezone abbreviation meant for output only. Read more about that in the article "Leap Seconds and What To Do With Them". However, if it is necessary you can create a DateTime object with a different timezone type. In this case you want a "type 2" timezone associated with the DateTime object. You do that by simply forcing that timezone abbreviation when instantiating a DateTime object:

$date = new DateTime('today GMT');
var_dump( $date );

which shows:

object(DateTime)#1 (3) {
  ["date"]=>
  string(19) "2012-02-29 16:32:16"
  ["timezone_type"]=>
  int(2)
  ["timezone"]=>
  string(3) "GMT"
}

Things like this also work:

$date = new DateTime( "GMT" );
$date->setDate( 2012, 2, 19 );
var_dump( $date );

And of course, this is not limited to GMT only:

$date = new DateTime( "EST" );
$date->setDate( 2012, 2, 19 );
var_dump( $date );

which shows:

object(DateTime)#1 (3) {
  ["date"]=>
  string(19) "2012-02-19 16:37:58"
  ["timezone_type"]=>
  int(2)
  ["timezone"]=>
  string(3) "EST"
}


As a reminder of the three different types of timezones that can be attached to DateTime objects:

  1. A UTC offset, such as in new DateTime( "2012-02-29 -0500" );

  2. A timezone abbreviation, such as in new DateTime( "2012-02-29 EST" );

  3. A timezone identifier, such as in new DateTime( "2012-02-29 America/Montreal" );

Please also be aware that only DateTime objects with "type 3" timezones attached to them will calculate correctly over Daylight Saving Time boundaries.

If you want to learn more about Dates and Times, and how to use them with PHP, please get a copy of my book "php|architect's Guide to Date and Time Programming".

Shortlink

This article has a short URL available: http://drck.me/gmt-9bx

Comments

Varnish seems to be picky about the format of the If-Modified-Since/Last-Modified request/response headers if it is to support 304 Not Modified.

On some projects I have had to create a GMT timestamp by doing something like this to make Varnish respond with a 304:

$date = new DateTime('@' . $someUnixTimestamp);
header('Last-Modified: ' . $date->format('D, d M Y H:i:s') . ' GMT');

An example of the formatted timestamp:

Mon, 27 Feb 2012 12:31:48 GMT



Where's the +1 button ?

Thanks Derick for this !

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