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:
-
A UTC offset, such as in
new DateTime( "2012-02-29 -0500" );
-
A timezone abbreviation, such as in
new DateTime( "2012-02-29 EST" );
-
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".
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 !
Shortlink
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