Return to Revisions

On the javascript side, I am not seeging any value in creating the string format for the offset that you are creating, as you just end up spending more code in PHP trying to get to a usable format. Consider keeping offset in minute-based javascript format or converting to second-based format.

I would strongly consider using PHP's DateTime and related classes (in this case specifically, DateTimeZone) rather than the procedural date_* functions. Over time I think you will find them much more powerful and flexible in usage. This also moves you away from the concept of resetting you default system timezone, which I don't think you want to overwrite here, but rather us as fallback.

Let's show an example usage:

// assume $_COOKIE['hp_time_offset'] contains a minute based offset like
// output by javascript Date().getTimezoneOffset()

$system_tz = date_default_timezone_get();
if(!empty($_COOKIE('hp_time_offset')) {
   $offset_min = (int)$_COOKIE['hp_time_offset'];
   $offset_sec = $offset_min * 60;
   // is current system in daylight savings?
   $is_dst = (int)date('I');
   // get timezone name
   $tz = timezone_name_from_abbr('', $offset_sec, $is_dst);
    try {
        $date_timezone = new DateTimeZone($tz);
    } catch (Exception $e) {
        error_log('Invalid timezone string of "' . $tz . '" passed from cookie.');
        $tz = $system_tz;
    }
}

if(empty($timezone)) {
    // I am not wrapping this in try-catch as if there is a failure here
    // I don't know what you want to do to recover (as this should not happen).
    // Feel free to catch if needed for your implementation
    $date_timezone = new DateTimeZone($tz);
}

return $date_timezone;
// or if you just want to return the timezone string
// return $date_timezone->getName();

This proposed functionality gives you a valid DateTimeZone object representing either the cookie-derived timezone or, as a fallback, the system timezone. You can use this object to instantiate any DateTime object you may need on your system with those objects automatically taking on the expected timezone behavior.