gmmktime

babonk at irc dot freenode dot net (2009-05-05 13:00:00)

GMT time is useful for avoiding the daylight's savings issue.  I had to use it to get a working date difference function:

<?php
function dateDiff($date1, $date2) //returns the difference, in days, between two dates.  avoids the daylight's savings issue by using GMT
{
    $date1 = date_parse($date1);
    $date2 = date_parse($date2);
    return ((gmmktime(0, 0, 0, $date1['month'], $date1['day'], $date1['year']) - gmmktime(0, 0, 0, $date2['month'], $date2['day'], $date2['year']))/3600/24);
}
?>

Thomas (2008-05-16 00:45:41)

I reviewed Greg's note and can confirm the different output of mktime() and gmmktime(). Ben stated that this true only if you use arguments. But this is not true. If you are outside of the GMT timezone mktime() is never identical to gmmktime().

ben at sixg dot com (2008-05-15 17:17:57)

As an addendum to my previous note (php.net said it was too long and I had to split this part off), I'll leave you with one more fun tidbit. :)
(See the note below this one for a full explanation of timezones versus unix timestamps.)
3 AM PST == 11 AM GMT, right? 11 AM _GMT_ > 3 AM _GMT_ (same timezone) right? So, logically, 3 AM PST > 3 AM GMT. (Makes sense right, it is 3 AM in California 8 hours after it is 3 AM in Greenwich.) Now, because timestamps are timezone-independent (or, always GMT/UTC), you get a strange situation. mktime(3) > gmmktime(3). Even though PST is GMT-8, so PST is less than GMT, mktime(3) is _greater_ than gmmktime(3). At first glance you'd probably expect mktime(3) to be less than gmmktime(3), but because of everything I described before, about how timestamps are always GMT/UTC (or, in some senses, no timezone at all), it ends up being "backwards." (Run a few examples yourself if you don't believe me.) Yet another reason not to try to deal with timezones at the timestamp level!

ben at sixg dot com (2008-05-15 16:38:55)

The interaction of timezones with unix timestamps is a bit tricky so I thought I'd clarify so that people aren't led too far astray by Greg's and Glen's comments. :)
In the POSIX standard, "unix time" does not have a specified timezone. It is universal. For most intents and purposes you can think of them as always being GMT/UTC. (And you can derive the UTC time from a "timestamp" by dividing it by 86400 and looking at the modulus.) Do not ever try to adjust a timestamp by a timezone offset (specifically, do not ever use the code at the end of Glen's note). Timezones are basically used only when "rendering" a timestamp from "unix time" into a "civil time" date/time string.
Let's take an example. PST is GMT-8, and EST is GMT-5. So when it is 3 AM in PST, it is 6 AM in EST. At that exact moment in time, the _timestamp_ is identical in both time zones. If I am sitting at my computer in PST, and you are at yours in EST, and I call you up and read you the current unix timestamp on my computer, it will match yours (assuming our clocks are both set accurately for our timezone).
So, time() always will return the same thing at the same actual moment, anywhere in the world. gmmktime() and mktime(), when given specific time parameters, convert those time parameters FROM the appropriate timezone (GMT for gmmktime(), local time for mktime()), before computing the appropriate timestamp. Again, for most intents and purposes you can imagine that mktime() first converts your input parameters to GMT and then calls gmmktime() which produces a GMT timestamp. (For the purposes of this explanation, please ignore the fact that the PHP documentation says that internally gmmktime() calls mktime().)
HOWEVER, when called with no arguments, gmmktime() uses the current GMT time, and mktime() uses the current local time. So, if you imagine the above conversion taking place where mktime() converts the (current) local time to GMT, it ends up essentially calling gmmktime() with the _current_ GMT time, just like gmmktime() does all by itself.
This is why time(), gmmktime(), and mktime() all return the same exact timestamp, _when called with no arguments_. This is why Glen saw them all produce the same thing.
Greg wrote that gmmktime() will return something different if you are not sitting in the GMT timezone, but this is only true if you have given it arguments from which to construct a timestamp.
So let's look at that situation again. Say I am in PST and it's 3 AM PST. (And therefore it is 11 AM GMT.) mktime() lets me override one field at at time, and the first argument is, conveniently, the hour field. So if I call mktime(3), I get the same answer as mktime(). Makes sense, right? I just told it to give me the timestamp corresponding to 3 AM local time. If I call gmmktime(11), I get the same answer as gmmktime(), since it is currently 11 AM GMT. mktime(3) and gmmktime(11) refer to the same exact point in time, because PST is 8 hours behind GMT. So it makes sense that mktime(3) == gmmktime(11). And sine mktime() == mktime(3) (at this moment), and gmmktime() == gmmktime(11) (at this moment), it makes sense that gmmktime() == mktime().
Now, back to Greg's note about gmmktime() being different. With identical arguments, their output will be different, e.g. mktime(3) != gmmktime(3). This makes sense because 3 AM PST != 3 AM GMT.
Okay, that should be all you need to know to deal with the interaction between timestamps and timezones. Don't ever try to convert timezones by adding or subtracting to the timestamp. Timestamps don't really have timezones, it is apples and oranges, and you'll either get the wrong answer in some situations or end up with code that no one can maintain. Leave it up to the higher-level PHP functions to do the conversion. (If you want to hack things, strtotime is handy and it can work with timezones; let it do the hard work for you.)

Greg (2008-04-17 14:19:57)

In addition to GLEN's post on DEC 5th 2007
(Just a note for others)
The results for the three time types Glen listed under "returns" all returned the exact same timestamp
- 1196812966
Using those three functions will only make the exact same timestamp if the server the code is executed on IS in GMT timezone.
'mktime()' and 'time()' will both return the timestamp for your server time, and will both be the same as eachother.
gmmktime will return a GMT timestamp.
So if run on a server that is NOT in the GMT timezone then gmmktime will return a different timestamp to the other two.

PHPcoder at freemail dot ig3 dot net (2007-09-06 11:26:27)

gmmktime() should ONLY be used to create a timestamp when specifying a specific GMT date and time.
If you want to make a valid time stamp for the current date & time, use mktime() instead.
UNIX timestamps, by definition, store the GMT time relative to the UNIX epoch.
gmmktime() (without any parameters specified) will effectively use the computer's LOCAL time values just the same as if they were explicit parameters, and the resulting time stamp will be incorrect. (The resulting timestamp will actually be offset in the OPPOSITE direction of the local timezone offset from GMT!)

moshe dot ortov at gmail dot com (2007-06-13 12:57:52)

Beware that despite the documentation which states is_dst is ignored, with PHP 5.2 at least, it is not actually ignored and will cause a 1 hour offset on the UTC time returned.

This caused some interesting bugs, especially with the tzdelta function shown from previous posts below - you need to make the final parameter a 0 instead of $ar[8] otherwise you get an off-by-1-hour as a result.

As a result, I now use :

<?php
function tzdelta ( $iTime = 0 ) {
        if ( 0 == $iTime ) { $iTime = time(); }
        $ar = localtime ( $iTime );
        $ar[5] += 1900; $ar[4]++;
        $iTztime = gmmktime ( $ar[2], $ar[1], $ar[0], $ar[4], $ar[3], $ar[5], 0);
        return ( $iTztime - $iTime );
}
?>

Tom (2007-01-19 02:05:28)

I was getting odd results using gmmktime...

I was converting a date/time to a timestamp to get a date/time in the future. Date/time to convert: Friday 19th of January 2007 10:06:00 PM.

<?php

gmmktime(9, 46, 0, 1 19, 2007);

?>

Returns this timestamp: 1169201160.

When formatted into a GMT date/time it comes out as Friday 19th of January 2007 10:06:00 AM. Notice it's 12 hours out from the original date/time.

I finally found the reason to be that gmmktime needs a 24 time not 12, should of noticed this since it doesn't have an am/pm input.

So the correct call would be something like...

<?php

if($date_time_ampm == "PM") $time_hours += 12;

gmmktime($time_hours, 46, 0, 1 19, 2007);

?>

francois at bonzon dot com (2005-12-21 12:34:22)

In response to mirko at example dot com is_est function, to be exact, the two lines defining $begin_time and $end_time should read:

<?php

$begin_time = gmmktime(1, 0, 0, 3, $begin_date, $Y);
$end_time = gmmktime(1, 0, 0, 10, $end_date, $Y);

?>

Because the clocks go forward, resp. back at 01.00 UTC, not at midnight.

mirko at example dot com (2005-10-31 04:33:38)

<?php

/**
 * Check if given time is during Europen Summer Time
 *
 * @link http://en.wikipedia.org/wiki/European_Summer_Time
 * @param int $time UTC timestamp (GMT)
 * @return boolean true if it is EST else false
 */
function is_est($time)
{
    // get year
    $Y = gmdate("Y", $time);
    
    // calc start / end dates and time for that year
    $begin_date = (31 - (5*$Y/4 + 4) % 7);
    $end_date = (31 - (5*$Y/4 + 1) % 7);
    $begin_time = gmmktime(0,0,0,  3,$begin_date,$Y);
    $end_time = gmmktime(0,0,0,  10,$end_date,$Y);
    
    // if it's in that period
    $is_dst = $time >= $begin_time && $time < $end_time;
    return $is_dst; 
}

?>

ranjeet at magnet-i dot com (2005-09-16 05:52:54)

<?php
function getTimeRemaining($timeZonePass,
$dateTimeUser,$ID,$table,$coloumnName)
{
global $configVars;

$timeZoneDefault=explode("_",$timeZonePass);

$timeZone=(substr($timeZoneDefault[0],1))*(60) ;

if((substr($timeZoneDefault[0],0,1)) =="+")
   $defaultSeconds=gmdate("Y-m-d-H-i-s",time()+
   $timeZone);
elseif((substr($timeZoneDefault[0],0,1)) =="-")
   $defaultSeconds=gmdate("Y-m-d-H-i-s",time()-
   $timeZone);
else
   $defaultSeconds=gmdate("Y-m-d-H-i-s",time());

$defaultSecondsExp=explode("-",$defaultSeconds);

$defaultGmktime=gmmktime($defaultSecondsExp[3],
           $defaultSecondsExp[4],
$defaultSecondsExp[5], $defaultSecondsExp[1],
           $defaultSecondsExp[2],
$defaultSecondsExp[0]);

$dateArray=explode("-",$dateTimeUser);

$slectedGmktime=gmmktime(23,59,59,$dateArray[1],
           $dateArray[2],$dateArray[0]);

if((substr($timeZoneDefault[0],0,1)) =="+")
   $slectedGmktimeAdded=($slectedGmktime)+
                       ($timeZone);
elseif((substr($timeZoneDefault[0],0,1)) =="-")
   $slectedGmktimeAdded=($slectedGmktime)+
                       ($timeZone);
else
   $slectedGmktimeAdded=($slectedGmktime);

//$slectedGmDate=gmdate("Y-m-d-H-i-s",
           $slectedGmktimeAdded);

$timeMinus=$slectedGmktimeAdded -
       $defaultGmktime;

$secondsInDay= 60*60*24;

if ($secondsInDay <= $timeMinus)
{
   $daysRemaining = floor($timeMinus/
                   $secondsInDay);
   if($daysRemaining >1 )
       return $daysRemaining ."&nbsp;Days&nbsp;";
   else
       return $daysRemaining ."&nbsp;Day&nbsp;";
}

if(empty($daysRemaining))
{   
   $secondsInHour= 60*60;
   if ($secondsInHour <= $timeMinus)
   {
       $hoursRemaining =floor ($timeMinus/
                       $secondsInHour);
       if($hoursRemaining > 1 )
           return $hoursRemaining ."&nbsp;Hours&nbsp;";
       else
           return $hoursRemaining ."&nbsp;Hour&nbsp;";
   }
}

if(empty($hoursRemaining))
{
   //$secondsRemaining = $timeMinus. "&nbsp;Seconds";
   $secondsInMinute = 60;
   if ($secondsInMinute <= $timeMinus)
   {
       $mintuesRemaining =floor ($timeMinus/
               $secondsInMinute);
       if($mintuesRemaining > 1 )
           return $mintuesRemaining ."&nbsp;Minutes&nbsp;";
       else
           return $mintuesRemaining ."&nbsp;Minute&nbsp;";
   }
}

if(empty($mintuesRemaining))
{
   $secondsRemaining = $timeMinus;
   if($secondsRemaining < 0)
   {   
       timedOutSale($ID,$table,$coloumnName);
       return "Time out";
   }
   elseif($secondsRemaining > 1 )
       return $secondsRemaining . "&nbsp;Seconds";
   else
       return $secondsRemaining . "&nbsp;Second";
}
}

function timedOutSale($ID,$table)
{
   GLOBAL $configVars, $db,$tableNames;
   $query =  "UPDATE " . $table
                   . " SET status = 'T'
                   WHERE $coloumnName = '" . $ID ."'";

           $result = $db->query($query);
   return;
}

?>

(2005-09-15 09:02:01)

<?php
function getTimeRemaining($timeZonePass,$dateTimeUser,$ID,$table,$coloumnName)
{
global $configVars;

$timeZoneDefault=explode("_",$timeZonePass);

$timeZone=(substr($timeZoneDefault[0],1))*(60) ;

if((substr($timeZoneDefault[0],0,1)) =="+")
    $defaultSeconds=gmdate("Y-m-d-H-i-s",time()+
    $timeZone);
elseif((substr($timeZoneDefault[0],0,1)) =="-")
    $defaultSeconds=gmdate("Y-m-d-H-i-s",time()-
    $timeZone);
else
    $defaultSeconds=gmdate("Y-m-d-H-i-s",time());

$defaultSecondsExp=explode("-",$defaultSeconds);

$defaultGmktime=gmmktime($defaultSecondsExp[3],
            $defaultSecondsExp[4],
$defaultSecondsExp[5], $defaultSecondsExp[1],
            $defaultSecondsExp[2],
$defaultSecondsExp[0]);

$dateArray=explode("-",$dateTimeUser);

$slectedGmktime=gmmktime(23,59,59,$dateArray[1],
            $dateArray[2],$dateArray[0]);

if((substr($timeZoneDefault[0],0,1)) =="+")
    $slectedGmktimeAdded=($slectedGmktime)+
                        ($timeZone);
elseif((substr($timeZoneDefault[0],0,1)) =="-")
    $slectedGmktimeAdded=($slectedGmktime)+
                        ($timeZone);
else
    $slectedGmktimeAdded=($slectedGmktime);

//$slectedGmDate=gmdate("Y-m-d-H-i-s",
            $slectedGmktimeAdded);

$timeMinus=$slectedGmktimeAdded - 
        $defaultGmktime;

$secondsInDay= 60*60*24;

if ($secondsInDay <= $timeMinus)
{
    $daysRemaining = floor($timeMinus/
                    $secondsInDay);
    if($daysRemaining >1 )
        return $daysRemaining ."?Days?";
    else
        return $daysRemaining ."?Day?";
}

if(empty($daysRemaining))
{    
    $secondsInHour= 60*60;
    if ($secondsInHour <= $timeMinus)
    {
        $hoursRemaining =floor ($timeMinus/
                        $secondsInHour);
        if($hoursRemaining > 1 )
            return $hoursRemaining ."?Hours?";
        else
            return $hoursRemaining ."?Hour?";
    }
}

if(empty($hoursRemaining))
{
    //$secondsRemaining = $timeMinus. "?Seconds";
    $secondsInMinute = 60;
    if ($secondsInMinute <= $timeMinus)
    {
        $mintuesRemaining =floor ($timeMinus/
                $secondsInMinute);
        if($mintuesRemaining > 1 )
            return $mintuesRemaining ."?Minutes?";
        else
            return $mintuesRemaining ."?Minute?";
    }
}

if(empty($mintuesRemaining))
{
    $secondsRemaining = $timeMinus;
    if($secondsRemaining < 0)
    {    
        timedOutSale($ID,$table,$coloumnName);
        return "Time out";
    }
    elseif($secondsRemaining > 1 )
        return $secondsRemaining . "?Seconds";
    else
        return $secondsRemaining . "?Second";
}
}

function timedOutSale($ID,$table)
{
    GLOBAL $configVars, $db,$tableNames;
    $query =  "UPDATE " . $table
                    . " SET status = 'T'
                    WHERE $coloumnName = '" . $ID ."'";

            $result = $db->query($query);
    return;
}
?>

jbr (AT) ya-right.com (2005-02-13 10:04:17)

I have seen many different hacked versions of this function for people using Windows that want to support dates before Jan 1, 1970. Here is yet another one that is different than others you may have seen! It does not use loops like all the others I have seen! 

<?php

// usage...

echo win_gmmktime ( 12, 43, 16, 07, 23, 1946 );

function win_gmmktime ( $hour, $minute, $second, $month, $day, $year )
{
    if ( $year > 1969 )
    {
        return ( gmmktime ( $hour, $minute, $second, $month, $day, $year ) );
    }

    $t = 0;
    $ds = 86400;
    $hs = 3600;
    $dy = 365;
    $ms = 60;

    $months = array ( 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 );
    $leap_year = $year % 4 == 0 && ( $year % 100 > 0 || $year % 400 == 0 ) ? true : false;

    if ( $year < 1969 )
    {
        $y = 1969 - $year;
        $t -= ( $y * $dy ) * $ds;
        $x = ceil ( $y / 4 );

        if ( $leap_year && $month > 2 )
        {
            $x -= 1;
        }

        $t -= $x * $ds;
    }

    if ( $month != 12 )
    {
        $tm = $months;
        $tm = array_slice ( $tm, $month );
        $t -= array_sum ( $tm ) * $ds;
        unset ( $tm );
    }

    $nh = ( ( $month == 2 && $leap_year ? 29 : $months[$month-1] ) - $day );
    $t -= $nh != 0 ? $nh * $ds : 0;
    $nh = 23 - $hour;
    $t -= $nh != 0 ? $nh * $hs : 0;
    $nh = 59 - $minute;
    $t -= $nh != 0 ? $nh * $ms : 0;
    $nh = 59 - $second;
    $t -= $nh != 0 ? $nh + 1 : 0;

    return ( $t );
}

?>

welch at advomatic dot com (2005-02-02 12:48:56)

Here is a handy routine for counting down to the minute, hour, and day to a timestamp

<?php
      $minutesleft = floor(($timestamp - gmtime()) / 60);

      if ($minutesleft < 0) {
        $timeleft = 'NOW';
      }
      else if ($minutesleft < 60) {
        $timeleft = ($minutesleft==1 ? '1 minute' : $minutesleft.' minutes');
      }
      else if ($minutesleft >= 60 && $minutesleft < (24*60)) {
        $timeleft = (floor($minutesleft/60) == 1 ? '1 hour' : floor($minutesleft/60).' hours');
      }
      else if ($minutesleft >= (24*60)) {
        $days = floor($minutesleft / (24*60));
        // hours remainder
        $hours = ($minutesleft % (24*60)) / 60;
        // hours left in the day
        $hours_left = ((time() / 60) % (24*60)) / 60;
        // see if the remainder of hours is greater than the hours left in today, if so increase the days by one so that the days remaining mimics the date rather than how many 24 hour periods there are between now and then.
        if($hours > $hours_left) {
          $days++;
        }
        $timeleft = ($days == 1 ? '1 day' : $days.' days');
      }

      echo $timeleft;
?>

imoore76 at yahoo dot com (2005-01-15 13:16:58)

Why not just do:

<?php

// assuming $start and $end are timestamps

$day_diff = floor(abs($start - $end) / 86400);

?>

justin at booleangate dot org (2005-01-05 00:10:23)

Here's a play on turgut85's countDays function.  I've found it to be more efficient and it accepts unix timestamps rather than arrays.  Thanks for the ideas.

<?php
function count_days($start, $end) {
  // Count the days between $start and $end where both $start and $end
  //  are UNIX timestamps
  
  // Swap the two values if end is greater than start (to avoid the
  //  loop of death).
  if ($start < $end) {
    $t = $start;
    $start = $end;
    $end = $t;
  }
  
  // Increment the start time by one day until it is equal to the
  //  end time
  
  $days = 0;
  while ( $start < $end ) {
    $start = strtotime("+1 days", $start);
    $days++;
  }
  
  return $days;
}
?>

turgut85 at hotmail dot com (2004-12-06 20:53:44)

<?php

// THIS ROUTINE COUNT DAYS BEETWEEN TWO DATE //
// BEGIN DATE COULD BE IN THE PAST OR IN THE FUTURE //

$beg['YEAR']=2004;
$beg['MONTH']=10;
$beg['DAY']=1;

$end['YEAR']=2004;
$end['MONTH']=10;
$end['DAY']=3;

function countDays ($beg,$end) {
    
$start = gmmktime(0,0,0,$beg['MONTH'],$beg['DAY'],$beg['YEAR']);
$endin = gmmktime(0,0,0,$end['MONTH'],$end['DAY'],$end['YEAR']);

// echo $start."\n";
// echo $endin."\n";

$day = 0;

if ($start < $endin) {
$toward = 1;
} else  {
$toward = 0;
}

$mover = $start;

if ($start != $endin) {
    
do {
    
$day++;
    
    if ($toward)  {
    $mover = gmmktime(0,0,0,$beg['MONTH'],($beg['DAY']+$day),$beg['YEAR']);
    } else {
    $mover = gmmktime(0,0,0,$beg['MONTH'],($beg['DAY']-$day),$beg['YEAR']);
    }

    
} while ($mover != $endin);

}

echo $day;

return $day;
}

echo countDays ($beg,$end). " days. ";

?>

Turgut Z. YESILYURT
turgut85@hotmail.com
System and Application Developer
New Jersey, USA

mwwaygoo AT hotmail DOT com (2004-11-22 09:36:16)

REF: http://www.dti.gov.uk/er/sumtimetb.htm
Since 1981 EC Directives have prescribed the start and end dates of summer time in all Member States. There have to date been eight Directives which have set summer-time arrangements for fixed periods. The Summer Time Act 1972 sets the appropriate dates in the UK and summer-time orders have been made as necessary to implement the European Directives. The 9th EC Directive prescribes the start and end dates of summer time as the last Sundays in March and October respectively. These dates are in line with those already operating in the United Kingdom. The 9th Directive provides that these start and end dates should apply indefinitely.
---
9th EC Directive - 19 January 2001
Article 1
For the purposes of this Directive "summer-time period" shall mean the period of the year during which clocks are put forward by 60 minutes compared with the rest of the year.
Article 2
From 2002 onwards, the summer-time period shall begin, in every Member State, at 1.00 a.m., Greenwich Mean Time, on the last Sunday in March.
Article 3
From 2002 onwards, the summer-time period shall end, in every Member State, at 1.00 a.m., Greenwich Mean Time, on the last Sunday in October.
You stand corrected ;-) (well up until 2007 anyway)

Andy (2004-10-19 07:00:13)

mwwaygoo's code isn't quite right. My understanding is that the relevant dates for changing between daylight saving time in the UK is the third sunday of march and october - not the last sunday!

mwwaygoo AT hotmail DOT com (2004-09-13 03:51:03)

I had a problem with hosting a UK site on a US server, the times didnt match (obviously) and also didnt account for daylight savings time. The daylight savings dates and times of change differ worldwide, so detecting if the server was in dst wouldnt work (see http://webexhibits.org/daylightsaving/).

Here is a function for creating a timestamp which can be used by date() to create all the parameters required to display the local time (site not server). I have used GMT time to create the timestamp as there is no offset for UK time (+00).

<?php
function UKdst_time()
{
    // created by Matthew Waygood (www.waygoodstuff.co.uk)
    $timestamp = mktime(gmdate("H, i, s, m, d, Y")); // UTC time
    $this_year=gmdate("Y", $timestamp);

    // last sunday in march at 1am UTC
    $last_day_of_march=gmmktime(1,0,0,3,31,$this_year);
    $last_sunday_of_march=strtotime("-".gmdate("w", $last_day_of_march)." day", $last_day_of_march);
    
    // last sunday in october at 1am UTC
    $last_day_of_october=gmmktime(1,0,0,10,31,$this_year);
    $last_sunday_of_october=strtotime("-".gmdate("w", $last_day_of_october)." day", $last_day_of_october);

    if( ($timestamp > $last_sunday_of_march) && ($timestamp < $last_sunday_of_october) )
    {
        $timestamp=$timestamp+3600; // foward one hour
    }
    return $timestamp;
}
?>

Dave (2004-03-29 09:35:17)

There appears to be a discrepency between PHP and C timestamps. The C time() and gettimeofday() functions are documented to return based on UTC time, but the value obtained doesn't match the PHP gmmktime() function. Instead, it matches the PHP mktime() function, which is supposed to be local time.

It seems that C always uses a UTC timestamp and adjusts to local time through different handling functions (gmtime() vs localtime()). PHP appears to use differing UTC/local timestamps, but single handling functions.
The exception to this rule is the PHP time() function, which appears to behave in the same was as the C version.
In short, if your PHP is working with timestamps created in C (or vice versa) make sure you are comparing apples to apples.

zhong311 at collegeclub dot com (2004-02-13 15:40:44)

When attempting to use HTTP's If-Modified-Since features for caching I ran into the problem of being able to compare the GMT date the browser was sending to my own Last-Modified date (stored in a database field). I saw many examples of how to create a GMT date from a unix timestamp, but little on how to actually get a GMT date into a unix timestamp. Perhaps someone has a better way, here's my solution:

<?php

function gmstrtotime($sgm) {
    $months = array(
      'Jan'=>1,
      'Feb'=>2,
      'Mar'=>3,
      'Apr'=>4,
      'May'=>5,
      'Jun'=>6,
      'Jul'=>7,
      'Aug'=>8,
      'Sep'=>9,
      'Oct'=>10,
      'Nov'=>11,
      'Dec'=>12
    );
    list($D, $d, $M, $Y, $H, $i, $s) = sscanf($sgm, "%3s, %2d %3s %4d %2d:%2d:%2d GMT");
    return gmmktime($H, $i, $s, $months[$M], $d, $Y);
}

// test: after all is said and done
// $time should be the same as $gmtime

$time = time();
$us = date("m/d/Y H:i:s",$time);
$sgm = gmdate("D, d M Y H:i:s",$time) . " GMT";

$gmtime = gmstrtotime($sgm);

echo $us . "<BR>";
echo $sgm . "<BR>";
echo $time . "<BR>";
echo $gmtime . "<BR>";

?>

My results:

02/13/2004 10:45:42
Fri, 13 Feb 2004 20:45:42 GMT
1076705142
1076705142

Credit to kyle at frozenonline dot com for his strtotime example