expires:
<?php
        $expires = strtotime('tomorrow', time());
        echo $expires;
        echo ' ('.strftime('%c', $expires).')';

?>

<br><br>

timediff (seconds until "expires"):
<?php
        $timediff = $expires - time();
        echo $timediff;
?>

<br><br>

filetime:
<?php
        $filetime = @filemtime('/filepath/tmp/element');
        echo $filetime;
        echo ' ('.strftime('%c', $filetime).')';
?>

<br><br>

<?php
        echo 'filetime + timediff: ';
        echo $filetime + $timediff;
        echo ' ('.strftime('%c', $filetime + $timediff).')';

        echo ' -- current time: ';
        echo time();
        echo ' ('.strftime('%c').')';
        echo ' -- ';
        echo ($filetime + $timediff < time()) ? 'expired' : 'not expired';
?>

<br><br>
<?php
        echo 'diff: ';
        echo $filetime + $timediff - time();
?>