It's really quite simple when you analyze it, I explain, what you need is to get the date as is, as you have stored it in your registry and then you must convert it to a unix format
$fecha_de_la_tabla=strtotime("2018-05-04 12:00:00");
then you must obtain the current date and convert it to unix format
$unix_actual=time();
Once you have this you must subtract the current unix time and the one you have stored in your table
$tiempo_transcurrido=$unix_actual-$fecha_de_la_tabla;
to finish you must use this function that I leave to convert the elapsed time in hours, minutes or seconds, as the case may be
function conversorSegundosHoras($tiempo_en_segundos) {
$horas = floor($tiempo_en_segundos / 3600);
$minutos = floor(($tiempo_en_segundos - ($horas * 3600)) / 60);
$segundos = $tiempo_en_segundos - ($horas * 3600) - ($minutos * 60);
$hora_texto = "";
if ($horas > 0 ) {
$hora_texto .= $horas . "h ";
}
if ($minutos > 0 ) {
$hora_texto .= $minutos . "m ";
}
if ($segundos > 0 ) {
$hora_texto .= $segundos . "s";
}
return $hora_texto;
}
echo conversorSegundosHoras($tiempo_transcurrido);