PHP timezone function not working - php

Question

I'm creating a forum, which also stores the time a post was sent, and I need to convert that into the user's timezone.
Now, the MySQL DataBase stores the time with UTC_TIMESTAMP() (in a column with the DATETIME type), and I created a little function from the code on http://www.ultramegatech.com/blog/2009/04/working-with-time-zones-in-php/ to convert the time to the user's timezone. This is the function:
function convertTZ($timestamp, $tz='UTC', $format='d-m-Y H:i:s') {
// timestamp to convert
$timestamp = strtotime($timestamp);
// the time formatting to use
$format = $format;
// the time zone provided
$tz = $tz;
// create the DateTimeZone object for later
$dtzone = new DateTimeZone($tz);
// first convert the timestamp into an RFC 2822 formatted date
$time = date('r', $timestamp);
// now create the DateTime object for this time
$dtime = new DateTime($time);
// convert this to the user's timezone using the DateTimeZone object
$dtime->setTimeZone($dtzone);
// print the time using your preferred format
$time = $dtime->format($format);
return $time;
}
And I made a test page at http://assets.momo40k.ch/timezones.php.
Now, when I insert a post into the DataBase at, say, 11:50 in my timezone (which is Europe/Rome), it inserts 09:50 in UTC, wich is correct, according to some online timezone converters.
But when I try to convert it back to Europe/Rome with the convertTZ() function, it returns 09:50, as if Europe/Rome is UTC. If I try converting it to a GMT+2:00 timezone, it returns 10:50. Can anyone fugure out why this is?
P.S: I'm not using the CONVERT_TZ() SQL function because my server does not support named timezones, so this function is my workaround.

Answer 1

Make sure your stored timestamps are UTC:
$date = new DateTime($timestamp, new DateTimeZone("UTC"));
$date->format(DATE_W3C); // does it gives the expected result ?
BTW your function can be simplified to this:
function convertTZ($timestamp, $tz='UTC', $format='d-m-Y H:i:s') {
$dtime = new DateTime($timestamp, new DateTimeZone("UTC"))
$dtime->setTimezone(new DateTimeZone("UTC"));
return $dtime->format($format);
}

Answer 2

MySQL always stores TIMESTAMP fields in UTC internally (that's the definition of a unix timestamp, in fact). So when you SELECT or UPDATE/INSERT/REPLACE, the time you get or set is always in the MySQL server's local time zone.
So a common mistake is to store UTC_TIMESTAMP(), which MySQL interprets as a local time and so the current time gets double-converted to UTC when it stores it internally in the field as a unix TIMESTAMP.