PHP & MySQL: Converting Stored TIMESTAMP into User's Local Timezone

Question

So I have a site with a comments feature where the timestamp of the comment is stored in a MySQL database. From what I understand, the timestamp is converted to UTC when stored, then converted back to the default timezone when retrieved. In my case, my server is in the Central Daylight Time timezone (CDT).

I have a plan to get the timezone from each user via entry form. I just wanted to know how to convert the TIMESTAMP value into the user's timezone.

• First, would I convert from UTC to local timezone? Or CDT to local timezone?
• Secondly, how would I go about doing that in PHP? Would I just do:

$userTimezone = new DateTimeZone($userSubmittedTimezoneString);
$myDateTime = new DateTime($storedTimestamp, $userTimezone);

...or is that not correct?

Answer 1

Date/time/datetime values are stored in MySQL as you supply them. I.e. if you INSERT the string 2012-04-17 12:03:23 into a DATETIME column, that's the value that will be stored. It will be converted internally into a timestamp which may or may not be accurate (see below), but when you query for the value again, you'll get the same value back out; the roundtrip is transparent.

Problems may occur if you try to do time calculations inside SQL. I.e. any operation that requires SQL to take the timezone and/or the server time into account. For example, using NOW(). For any of those operations, the timezone and/or server time should be set correctly. See Time Zone Problems.

If that doesn't concern you and you only need to do calculations in PHP, you only need to make sure you know from which timezone to which timezone you want to convert. For that purpose it can be convenient to standardize all times to UTC, but it is not necessary, as timezone conversions from any timezone to any other timezone work just as well, as long as you're clear about which timezone you're converting from and to.

date_default_timezone_set('Asia/Tokyo'); // your reference timezone here

$date = date('Y-m-d H:i:s');

/* INSERT $date INTO database */;

$date = /* SELECT date FROM database */;

$usersTimezone = new DateTimeZone('America/Vancouver');
$l10nDate = new DateTime($date);
$l10nDate->setTimeZone($usersTimezone);
echo $l10nDate->format('Y-m-d H:i:s');