Move is_a_date_arg to DateTimeFormat::isYearMonth

- Improved functionality
- Added tests

tests/src/Util/DateTimeFormatTest.php

@@ -0,0 +1,61 @@
+<?php
+
+namespace Friendica\Test\src\Util;
+
+use Friendica\Test\MockedTest;
+use Friendica\Util\DateTimeFormat;
+
+class DateTimeFormatTest extends MockedTest
+{
+	public function dataYearMonth()
+	{
+		return [
+			'validNormal' => [
+				'input' => '1990-10',
+				'assert' => true,
+			],
+			'validOneCharMonth' => [
+				'input' => '1990-1',
+				'assert' => true,
+			],
+			'validTwoCharMonth' => [
+				'input' => '1990-01',
+				'assert' => true,
+			],
+			'invalidFormat' => [
+				'input' => '199-11',
+				'assert' => false,
+			],
+			'invalidFormat2' => [
+				'input' => '1990-15',
+				'assert' => false,
+			],
+			'invalidFormat3' => [
+				'input' => '99-101',
+				'assert' => false,
+			],
+			'invalidFormat4' => [
+				'input' => '11-1990',
+				'assert' => false,
+			],
+			'invalidFuture' => [
+				'input' => '3030-12',
+				'assert' => false,
+			],
+			'invalidYear' => [
+				'input' => '-100-10',
+				'assert' => false,
+			],
+		];
+	}
+
+	/**
+	 * @dataProvider dataYearMonth
+	 */
+	public function testIsYearMonth(string $input, bool $assert)
+	{
+		$dtFormat = new DateTimeFormat();
+
+		$this->assertEquals($assert, $dtFormat->isYearMonth($input));
+	}
+}