Use the mktime function, and do the backwards math.
//mktime(hour,minutes,seconds,month,date,year);
$thursday = mktime(0,0,0,6,20,2002);
$secs_till = ($thursday - time());
$days_till = ($thursday / (60 60 24));
$hours_till = ($thursday / (60 * 60));
$mins_till = ($thursday / (60));