Description:
------------
use DateTime() parse date error,such as Ym type:201501 、201512

I don't know user will input which time type,so i can't define a format like
DateTime::createFromFormat('Ym', '201501')

Test script:
---------------
<?php

$input = '201501';
$output = (new DateTime($input))->format('Ym');
echo $output;

Expected result:
----------------
201501

Actual result:
--------------
201801