Time difference time() in php

<?php

date_default_timezone_set("Asia/Calcutta");

$up_from = 1617708670;

echo 'UP from : '.date('H:i:s', $up_from);

echo '<hr>';

$last_down = 1617708488;

$diff = $up_from - $last_down;

echo 'Last Down : '.date('H:i:s', $last_down).'<br>';

echo '<hr>';

$totalSec = abs($diff);

$downTime= gmdate("H:i:s", $totalSec);

echo 'Total Down Tome: '.$downTime;

echo '<hr>';

$init = 685;

$hours = floor($init / 3600);

$minutes = floor(($init / 60) % 60);

$seconds = $init % 60;

$totalDown = '';

if($hours){

$totalDown = "$hours hours $minutes minutes $seconds seconds";

}elseif($minutes){

$totalDown = "$minutes minutes $seconds seconds";

}else{

$totalDown = "$seconds seconds";

}

//4 minutes and 59 seconds).

echo $totalDown;

?>