How do I get Greenwich Mean Time in PHP?

Question

I have a server which is set to EST, and all records in the database are set to EST. I would like to know how to set it to GMT. I want to offer a time zone option to my users.

Answer 1

I would strongly suggest avoiding messing with UNIX timestamps to make it look like a different time zone. This is a lesson I've learnt the hard way, way too many times.

A timestamp is the number of seconds since Midnight 1 January 1970, GMT. It doesn't matter where you are in the world, a given timestamp represents the exact same moment in time, regardless of time zones. Yes, the timestamp "0" means 10am 1/1/70 in Australia, Midnight 1/1/70 in London and 5pm 31/12/69 in LA, but this doesn't mean you can simply add or subtract values and guarantee accuracy.

The golden example which stuffed me up every year for way too long was when daylight savings would crop up. Daylight savings means that there are "clock times" which don't exist in certain parts of the world. For example, in most parts of the US, there was no such time as 2:01am on April 2, 2006.

Enough quasi-intelligible ranting though. My advice is to store your dates in the database as timestamps, and then...

• If you need the local time at the server, use date()
• If you need to get GMT, then use gmdate()
• If you need to get a different timezone, use date_default_timezone_set() and then date()

For example,

$timestamp = time();

echo "BRISBANE: " . date('r', $timestamp) . "\n";
echo "     UTC: " . gmdate('r', $timestamp) . "\n";
date_default_timezone_set('Africa/Johannesburg');
echo "  JOBURG: " . date('r', $timestamp) . "\n";

// BRISBANE: Tue, 12 May 2009 18:28:20 +1000
//      UTC: Tue, 12 May 2009 08:28:20 +0000
//   JOBURG: Tue, 12 May 2009 10:28:20 +0200

This will keep your values clean (you don't have to be worrying about adding offsets, and then subtracting before saving), it will respect all Daylight Savings rules, and you also don't have to even look up the timezone of the place you want.

Answer 2

No matter in which GMT time zone the server is, here is an extremely easy way to get time and date for any time zone. This is done with the time() and gmdate() functions. The gmdate() function normally gives us GMT time, but by doing a trick with the time() function we can get GMT+N or GMT-N, meaning we can get the time for any GMT time zone.

For example, if you have to get the time for GMT+5, you can do it as follows

<?php 
  $offset=5*60*60; //converting 5 hours to seconds.
  $dateFormat="d-m-Y H:i";
  $timeNdate=gmdate($dateFormat, time()+$offset);
?>

Now if you have to get the time for GMT-5, you can just subtract the offset from the time() instead of adding to it, like in the following example where we are getting the time for GMT-4

<?php 
  $offset=4*60*60; //converting 4 hours to seconds.
  $dateFormat="d-m-Y H:i"; //set the date format
  $timeNdate=gmdate($dateFormat, time()-$offset); //get GMT date - 4
?>

Answer 3

Here is a list of the time zones with different from GMT, hope this is helpful!

echo '<pre>';
$zones_array = array();        
$timestamp = time();         
# to maintain the original timezone, store before
$default_timezone = date_default_timezone_get();
foreach (timezone_identifiers_list() as $key => $zone) {    
    date_default_timezone_set($zone);
    $zones_array[$key]['zone'] = $zone;
    $zones_array[$key]['diff_from_GMT'] = date('P', $timestamp);
}
# to maintain the original timezone, re-set after
date_default_timezone_set($default_timezone);    
print_r($zones_array);

Thanks!

Answer 4

It is important to note that if you are using date('Z') to convert a date that is stored in a database that you give the date the timestamp as the second argument, otherwise your result will not be accurate.

e.g.

In the UK, sometimes date('Z') will give 3600, sometimes it will give 0.

echo date('Z', strtotime('2012-01-01'));

Gives 0

echo date('Z', strtotime('2012-07-01'));

Gives 3600

So purely using strtotime($dbDateValue) - date('Z') can be wrong

Instead use:

$dbDateTimestamp = strtotime($dbDateValue); // where $dbDateValue something like 2012-01-01 22:34:00

$utcTimestamp = strtotime($dbDateTimestamp) - date('Z', $dbDateTimestamp);

This of course relies on PHP and the database both being set to the same timezone.

Answer 5

I was inspired by this question to write an alternative function to convert a timestamp to a needed timezone.

So, CORRECT procedure steps to record, display (convert) time:

1) record timestamp with time()
2) If you need to get a different timezone, use:
   2.1) Class based:
       2.1.1) DateTime class + setTimezone
       or
       2.1.2) date_default_timezone_set() and then date()
       
       2.1.1) is better and more flexible than 2.1.2)

    Function:
    
      function getLTime_class($ts, $uTZ, $format_str="Y.m.d H:i", $sTZ="America/Toronto"){
        // $ts - timestamp recorded with time(); if have date => convert to timestamp: strtotime($date_input); //for ex strtotime('2012-02-17 12:00:00'); => 1345219200 - same value for summer and winter (for toronto timezone);
        // $uTZ - TZ string (ex: 'America/Toronto'); convert timestamp to this TimeZone
        // $sTZ - TZ string (ex: 'America/Toronto'); convert timestamp from this TimeZone
        $TimeZone_server=new DateTimeZone($sTZ);
        $date_obj=new DateTime("@$ts", $TimeZone_server);
        $TimeZone_local=new DateTimeZone($uTZ);
        $date_obj->setTimeZone($TimeZone_local);
        return $date_obj->format($format_str);
      }

    Usage:
    
     $date_input=$_POST['date_input']; //read from POST
     $ts=strtotime($date_input); //for ex strtotime('2012-02-17 12:00:00'); => 1345219200 - same value for summer and winter (for toronto timezone);
     $userTZ='America/Toronto'; //read from user params
     $result=getLTime_class($ts, $userTZ, 'Y-m-d H:i:s');


   2.2) Non-Class based:

Function:

      //this function replaces the function with DateTime class. It's faster. $uTZoffset is integer.
      function getLTime_nonclass($ts, $uTZoffset, $format_str="Y.m.d H:i"){
        // $ts - timestamp recorded with time(); if have date => convert to timestamp: strtotime($date_input); //for ex strtotime('2012-02-17 12:00:00'); => 1345219200 - same value for summer and winter (for toronto timezone);
        // $uTZoffset - TZ offset (integer) $uTZoffset must consider DTS (for ex: in summer $uTZoffset=-4; in winter $uTZoffset=-5)
        $ts_offset=date('Z',$ts);
        if ($uTZoffset) $add_offset=$ts_offset-date('Z'); //when not UTC
        else $add_offset=0; //when UTC
        return date($format_str,$ts-$ts_offset+$uTZoffset*3600+$add_offset);
      }

    Usage:
    
     $date_input=$_POST['date_input']; //read from POST
     $ts=strtotime($date_input); //for ex strtotime('2012-02-17 12:00:00'); => 1345219200 - same value for summer and winter (for toronto timezone);
     $uTZoffset=-4; //read from user params. $uTZoffset - TZ offset (integer) $uTZoffset must consider DTS (for ex: in summer $uTZoffset=-4; in winter $uTZoffset=-5)
     $result=getLTime_nonclass($ts, $uTZoffset, 'Y-m-d H:i:s');

Results:

$date_input = 2012-08-17 12:00:00

Server date: summer (2013-08-26)
    getLTime_class      =>  $userTZ='America/Toronto'   =>  2012-08-17 12:00:00
    getLTime_nonclass   =>  $uTZoffset=-4               =>  2012-08-17 12:00:00
    getLTime_class      =>  $userTZ='UTC'               =>  2012-08-17 16:00:00
    getLTime_nonclass   =>  $uTZoffset=0                =>  2012-08-17 16:00:00

Server date: winter (2013-02-26)
    getLTime_class      =>  $userTZ='America/Toronto'   =>  2012-08-17 12:00:00
    getLTime_nonclass   =>  $uTZoffset=-5               =>  2012-08-17 12:00:00
    getLTime_class      =>  $userTZ='UTC'               =>  2012-08-17 16:00:00
    getLTime_nonclass   =>  $uTZoffset=0                =>  2012-08-17 16:00:00
    

$date_input = 2012-02-17 12:00:00

Server date: summer (2013-08-26)
    getLTime_class      =>  $userTZ='America/Toronto'   =>  2012-02-17 12:00:00
    getLTime_nonclass   =>  $uTZoffset=-4               =>  2012-02-17 12:00:00
    getLTime_class      =>  $userTZ='UTC'               =>  2012-02-17 17:00:00
    getLTime_nonclass   =>  $uTZoffset=0                =>  2012-02-17 17:00:00

Server date: winter (2013-02-26)    
    getLTime_class      =>  $userTZ='America/Toronto'   =>  2012-02-17 12:00:00
    getLTime_nonclass   =>  $uTZoffset=-5               =>  2012-02-17 12:00:00
    getLTime_class      =>  $userTZ='UTC'               =>  2012-02-17 17:00:00
    getLTime_nonclass   =>  $uTZoffset=0                =>  2012-02-17 17:00:00 

I decided to getLTime_nonclass hoping to make the conversion faster than using classes. All I ask is to confirm that getLTime_nonclass is a valid replacement for getLTime_class.

Answer 6

A lot of time there is task when necessary record time in DB in GMT+0, but function time() will return time of the server. Its possible resolve by next function:

/**
     * Converts a local Unix timestamp to GMT
     *
     * @param   int Unix timestamp
     * @return  int
     */
    function local_to_gmt($time = '')
    {
        if ($time === '')
        {
            $time = time();
        }

        return mktime(
            gmdate('G', $time),
            gmdate('i', $time),
            gmdate('s', $time),
            gmdate('n', $time),
            gmdate('j', $time),
            gmdate('Y', $time)
        );
    }