<?php
declare(strict_types=1);

$start = new DateTimeImmutable("2026-10-10 09:00:00", new DateTimeZone("UTC"));
$due = $start->add(new DateInterval("P7D"));
echo $start->format("Y-m-d H:i T"), "\n";
echo $due->format("Y-m-d H:i T"), "\n";