Dates Problem

<?php
if(isset($_GET['commands']) && isset($_GET['format']) && isset($_GET['date'])){
    $date = $_GET['date'];
    $outputFormat = $_GET['format'];
    $commands = $_GET['commands'];

    $date = DateTime::createFromFormat('d/m/Y', $date);

    $commands = array_map('trim', explode("\n", $commands));

    for ($i = 0; $i < count($commands); $i++) {
        $commandArgs = explode(" ", $commands[$i]);
        $value = $commandArgs[1];
        $command = $commandArgs[0];

        if($command == "add"){
            $date -> add(new DateInterval("P".$value."D"));
        }
        else{
            $date -> sub(new DateInterval("P".$value."D"));
        }
    }

    echo $date->format($outputFormat);
}