Twice a year, mktime(0,0,0,$month,$day,$year) returns 3600 seconds too much/to few seconds, as the code below shows. This error is reproduced on PHP 3.??.

...
0922658400 1999-03-29 -3600
0941410800 1999-11-01 3600
0954108000 2000-03-27 -3600
0972860400 2000-10-30 3600
0985557600 2001-03-26 -3600
1004310000 2001-10-29 3600
1017612000 2002-04-01 -3600
1035759600 2002-10-28 3600
1049061600 2003-03-31 -3600
1067209200 2003-10-27 3600
1080511200 2004-03-29 -3600
1099263600 2004-11-01 3600
...

<html><body><?php

$prev = 0;
for ($year=1970 ; $year < 2038 ; $year++)
{
  for ($month=1 ; $month <= 12; $month++)
  {
    switch ($month)
    {
    case 4: case 6: case 9: case 11:
      $days = 30;
      break;
    case 2:
      $days = (($year % 4 == 0) && ($year % 100 != 0) ? 29 : ($year % 400 == 0) ? 29 : 28);
      break;
    default:
      $days = 31;
    }
    for ($day=1 ; $day <= $days; $day++)
    {
      $date = mktime(0,0,0,$month,$day,$year);
      if ( $prev != 0 && $prev + 86400 != $date )
      {
        printf("%010d %d-%02d-%02d %d<br>\n",$date,$year,$month,$day,$date-$prev-86400);
      }
      $prev = $date;
    }
  }
}

?></body>