Description:
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From manual page: https://php.net/datetime.createfromformat
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PHP VER:

PHP 7.1.26-1+ubuntu18.04.1+deb.sury.org+1 (cli) (built: Jan 11 2019 14:13:49) ( NTS )
Copyright (c) 1997-2018 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2018 Zend Technologies
    with Zend OPcache v7.1.26-1+ubuntu18.04.1+deb.sury.org+1, Copyright (c) 1999-2018, by Zend Technologies
    with Xdebug v2.6.0, Copyright (c) 2002-2018, by Derick Rethans

When I use "Y" as the first argument and pass the value of two numbers as the second argument, function executed without errors and returns a DateTime object. Year in this DateTime object takes the value "00 + two numbers that I have passed"

Test script:
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<?php
$a = \DateTime::createFromFormat('m/d/Y', '12/31/29');
echo $a->format('Y-m-d') . PHP_EOL;

$a = \DateTime::createFromFormat('Y', '10');
echo $a->format('Y-m-d') . PHP_EOL;