2018 breaks DateTime tests

[criztovyl]

This is:

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

What is the expected behavior?

DateTime-Tests pass.

What is the current behavior?

DatTime-Tests fail because the test values are for 2017 (?).

PhpSpreadsheet/tests/data/Calculation/DateTime/DATEVALUE.php

Lines 162 to 186 in bae2348

	// 01/01 of the current year
	[
	42736,
	'1 Jan',
	],
	// 31/12 of the current year
	[
	43100,
	'31/12',
	],
	// Excel reads as 1st December 1931, not 31st December in current year
	[
	11658,
	'12/31',
	],
	// 05/07 of the current year
	[
	42921,
	'5-JUL',
	],
	// 05/07 of the current year
	[
	42921,
	'5 Jul',
	],

What are the steps to reproduce?

git clone https://github.com/PHPOffice/PhpSpreadsheet.git
cd PhpSpreadsheet
composer install
./vendor/bin/phpunit

Which versions of PhpSpreadsheet and PHP are affected?

No released version is affected, it's just a failing test.