date('Y-m-d H:i:s', strtotime($Site->getUTCOffset() . ' hours', strtotime(date($utcDate))));

<?php
/*
Example usage:
  $unixtime = TimeUtil::dateTimeToTimestamp('2009-04-01 15:36:13');
  echo TimeUtil::UTCToPST("M d, Y - H:i:s", $unixtime);
*/

// You should move this to your regular init method
date_default_timezone_set('UTC'); // make this match the server timezone

class TimeUtil {
    public static function timestampToDateTime($timestamp) {
        return gmdate('Y-m-d H:i:s', $timestamp);
    }

    public static function dateTimeToTimestamp($dateTime) {
        // dateTimeToTimestamp expects MySQL format
        // If it gets a fully numeric value, we'll assume it's a timestamp
        // You can comment out this if block if you don't want this behavior
        if(is_numeric($dateTime)) {
            // You should probably log an error here
            return $dateTime;
        }
        $date = new DateTime($dateTime);
        $ret = $date->format('U');
        return ($ret < 0 ? 0 : $ret);
    }

    public static function UTCToPST($format, $time) {
        $dst = intval(date("I", $time));
        $tzOffset = intval(date('Z', time()));
        return date($format, $time + $tzOffset - 28800 + $dst * 3600);
    }

}

<?php

$mysqlDate = '2009-04-01 15:36:13';

$dateTime = new DateTime ($mysqlDate);
$dateTime->setTimezone(new DateTimeZone('America/Los_Angeles'));

?>

function convert_time_zone($date_time, $from_tz, $to_tz)
    {
    $time_object = new DateTime($date_time, new DateTimeZone($from_tz));
    $time_object->setTimezone(new DateTimeZone($to_tz));
    return $time_object->format('Y-m-d H:i:s');
    }

mysql> set time_zone='America/New_York';

ALTER TABLE mytable CHANGE COLUMN Created Created timestamp NULL DEFAULT 0;

<?php

  function getNoteDateTimeZone($date = null, $from_dtz = 'US/Central', $to_dtz = null) {
        //$from_zt = 'US/Central'; // Time Zone = -06:00
        if (is_null($date) == FALSE && is_null($from_dtz) == FALSE && is_null($to_dtz) == FALSE) {
            // set TimeZone from
            $time_object = new DateTime($date, new DateTimeZone($from_dtz));
            $time_now_object = new DateTime("now", new DateTimeZone($from_dtz));
            // Change TimeZone
            $time_object->setTimezone(new DateTimeZone(trim($to_dtz)));
            $time_now_object->setTimezone(new DateTimeZone(trim($to_dtz)));
            // Is day = day in $time_now_object, $time_object..?
            if ($time_now_object->format('d') == $time_object->format('d')) {
                return $time_object->format('H:i:s');
            } else {
                return $time_object->format('Y-m-d H:i:s');
            }
        } else {
            return '';
        }
    }
?>

<?php
 $date = '2008-06-02 20:32:46';
 $dtz = 'America/Argentina/Buenos_Aires';
 echo getNoteDateTimeZone($date, 'US/Central', $dtz); // Out = 2008-06-02 23:32:46
?>

ADDTIME($utcDate,$Site->getUTCOffset())

function convert_to_user_date($date, $userTimeZone = 'America/Los_Angeles', $serverTimeZone = 'UTC', $format = 'n/j/Y g:i A')
{
    $dateTime = new DateTime ($date, new DateTimeZone($serverTimeZone));
    $dateTime->setTimezone(new DateTimeZone($userTimeZone));
    return $dateTime->format($format);
}

function convert_to_server_date($date, $userTimeZone = 'America/Los_Angeles', $serverTimeZone = 'UTC', $format = 'n/j/Y g:i A')
{
    $dateTime = new DateTime ($date, new DateTimeZone($userTimeZone));
    $dateTime->setTimezone(new DateTimeZone($serverTimeZone));
    return $dateTime->format($format);
}