How to get time diff in secs in WordPress?

I created a custom form on a WordPress website that lets users click a button to start recording time, and then later on another page there is a different form that lets the user stop recording time.

When they submit the first form to start recording time, I add this post meta to the database:

/* Start timer */
$now = strtotime('now');
update_post_meta( $post_id, 'start_time', $now);

An example value in the database is something like this: 1627607870

When they submit the second form later to stop recording time, I add this post meta to the database:

/* End timer */
$now = strtotime('now');
update_post_meta( $post_id, 'end_time', $now);

An example value in the database is something like this: 1627607890

To calculate how much time had passed, I do simple subtraction:

/* Total time */
$start_time     = get_post_meta($post_id, 'start_time', TRUE);
$end_time       = get_post_meta($post_id, 'end_time', TRUE);
$total_time     = $end_time - $start_time;
echo $total_time;

…which, using the example values shown above $total_time would return 20.

This all works fine – for me. But, I have some users in different countries who say that the total time returned is completely wrong, sometimes even showing what appears to be long completely random numbers.

So I am wondering if there something wrong with my approach? My gut feeling is that different timezones could affect $total_time but I might be wrong.