This code in CakePHP 3:

<?php

$date = '31/01/1973';
$ymd = DateTime::createFromFormat('!d/m/Y', $date)->format('Y-m-d');

Causes this error

Error: Class 'App\Controller\DateTime' not found

This code fixes it:

<?php

$date = '31/01/1973';
$ymd = \DateTime::createFromFormat('!d/m/Y', $date)->format('Y-m-d');

Apparently the \ backslash escapes the namespace and instead of it looking for DateTime in the current namespace it looks for it in the Global Namespace