Дмитрий

<?php
    $date = new DateTime('18.07.2019 15:17:34');
    echo "Исходная:     ", $date->format('d.m.Y H:i:s'), PHP_EOL;
    $date->modify('+ 5 day');
    echo "Через 5 дней: ", $date->format('d.m.Y H:i:s'), PHP_EOL;

Исходная:     18.07.2019 15:17:34
Через 5 дней: 23.07.2019 15:17:34

$timestamp = 1563452254;
$date_start=date("d.m.Y H:i:s", $timestamp);
$date_end=date("d.m.Y H:i:s", strtotime('+5 day',$timestamp));

echo $date_start.PHP_EOL; // 18.07.2019 12:17:34
echo $date_end.PHP_EOL; // 23.07.2019 12:17:34

$date = 1563452254;
$date_plus_five = $date + 60*60*24*5;
$date_normal = date("d.m.Y H:i:s", $date);
$date_plus_five_normal = date("d.m.Y H:i:s", $date_plus_five);
echo "$date_normal : $date_plus_five_normal";