Caculation of DATEDIF result is partially wrong

[albanduval]

This is:

- [x] a bug report
- [ ] a feature request
- [ ] **not** a usage question (ask them on https://stackoverflow.com/questions/tagged/phpspreadsheet or https://gitter.im/PHPOffice/PhpSpreadsheet)

What is the expected behavior?

DATEDIF calculated result should return the same result as MS Excel

Diff in years ("Y" unit) :

• 2000-02-02 => 2001-02-01 : should return 0
• 2001-05-04 => 2020-05-03 : should return 18

Diff in months with years ignored ("YM' unit) :

• 2000-02-02 => 2001-02-01 : should return 11
• 2001-05-04 => 2020-05-03 : should return 11

What is the current behavior?

Diff in years ("Y" unit) :

• 2000-02-02 => 2001-02-01 : returns -1
• 2001-05-04 => 2020-05-03 : return 17

Diff in months with years ignored ("YM' unit) :

• 2000-02-02 => 2001-02-01 : return -1
• 2001-05-04 => 2020-05-03 : return -1

What are the steps to reproduce?

<?php

require_once __DIR__ . '/vendor/autoload.php';

$intervals = [
    [ 'start' => '2000-02-02', 'end' => '2001-02-01' ],
    [ 'start' => '2001-05-04', 'end' => '2020-05-03' ],
    ];

foreach($intervals as $interval) {
    echo "{$interval['start']} => {$interval['end']}\n";
    $startDate = new \DateTime($interval['start']);
    $endDate = new \DateTime($interval['end']);

    $diffY = \PhpOffice\PhpSpreadsheet\Calculation\DateTime::DATEDIF($startDate, $endDate, 'Y');
    echo "  >  $diffY years\n";

    $diffYM = \PhpOffice\PhpSpreadsheet\Calculation\DateTime::DATEDIF($startDate, $endDate, 'YM');
    echo "  >  $diffYM months\n";
}

Which versions of PhpSpreadsheet and PHP are affected?

master branch since at least 2016