<!-- Collection of code snippets by Arne Vajhøj -->
<!-- posted to eksperten.dk, usenet and other places (2002-now) -->
<?php
$dt = new DateTime();
$dt->setDate(2018, 1, 31);
$dt->add(new DateInterval('P1M'));
$dt->sub(new DateInterval('P1M'));
echo $dt->format('d/m/Y');
?>