The date/time format used in HTTP headers

As you can see here, Last-Modified header has datetimes in RFC2616 format.

In section 14.29 Last-Modified you can see that date format should be:

"Last-Modified" ":" HTTP-date

An example of its use is

Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

Another quote from RFC2616 ^{read more} :

All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception.

In PHP you can use format D, d M Y H:i:s T if you use function gmdate() which always returns datetime in GMT offset/timeszone:

echo gmdate('D, d M Y H:i:s T');

If you wish to use DateTime extension:

$dt = new DateTime('UTC');
#$dt = new DateTime('2013-01-01 12:00:00', new DateTimezone('UTC'));
echo $dt->format('D, d M Y H:i:s \G\M\T');