Du bör läsa in värdet i sessionsvariabeln och använda det efteråt:
<?php
session_start();
if (isset($_SESSION['targetdate'])) {
// session variable_exists, use that
$targetDate = $_SESSION['targetdate'];
} else {
// No session variable, red from mysql
$result=mysql_query("select * from test where testid='29' LIMIT 1");
$time=mysql_fetch_array($result);
$dateFormat = "d F Y -- g:i a";
$targetDate = time() + ($time['duration']*60);
$_SESSION['targetdate'] = $targetDate;
}
$actualDate = time();
$secondsDiff = $targetDate - $actualDate;
$remainingDay = floor($secondsDiff/60/60/24);
$remainingHour = floor(($secondsDiff-($remainingDay*60*60*24))/60/60);
$remainingMinutes = floor(($secondsDiff-($remainingDay*60*60*24)- ($remainingHour*60*60))/60);
$remainingSeconds = floor(($secondsDiff-($remainingDay*60*60*24)- ($remainingHour*60*60))-($remainingMinutes*60));
$actualDateDisplay = date($dateFormat,$actualDate);
$targetDateDisplay = date($dateFormat,$targetDate);
?>