$startDate = 'now';
$endDate = '2013-08-12 14:30:00';

function date_diff($date1, $date2) {
    $diff = strtotime($date2) - strtotime($date1);
    return abs($diff);
}

function seconds2human($ss) {
    $s = $ss % 60;
    $m = floor(($ss % 3600) / 60);
    $h = floor(($ss) / 3600);

    return "$h hours, $m minutes, $s seconds";
}

echo "This file will expire in  ". seconds2human(date_diff($startDate,$endDate));