Date/Time: Make get_permalink() more resilient against PHP timezone changes.

Overriding default PHP timezone with `date_default_timezone_set()`, while not recommended, should not inadvertently result in changing existing permalinks.

Add a unit test.

Props Rarst, steevithak, archon810, maciejmackowiak, Ov3rfly, Cybr, hometowntrailers, scvleon, miette49.
Fixes #48623.

git-svn-id: https://develop.svn.wordpress.org/trunk@46795 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/date/getPermalink.php

@@ -0,0 +1,39 @@
+<?php
+
+/**
+ * @group date
+ * @group datetime
+ * @group post
+ */
+class Tests_Date_Get_Permalink extends WP_UnitTestCase {
+
+	function tearDown() {
+		delete_option( 'permalink_structure' );
+		update_option( 'timezone_string', 'UTC' );
+		date_default_timezone_set( 'UTC' );
+
+		parent::tearDown();
+	}
+
+	/**
+	 * @ticket 48623
+	 */
+	public function test_should_return_correct_date_permalink_with_changed_time_zone() {
+		$timezone = 'America/Chicago';
+		update_option( 'timezone_string', $timezone );
+		update_option( 'permalink_structure', '/%year%/%monthnum%/%day%/%hour%/%minute%/%second%' );
+		date_default_timezone_set( 'UTC' );
+
+		$post_id = self::factory()->post->create(
+			array(
+				'post_date'     => '2018-07-22 21:13:23',
+				'post_date_gmt' => '2018-07-23 03:13:23',
+			)
+		);
+
+		$this->assertEquals( 'http://example.org/2018/07/22/21/13/23', get_permalink( $post_id ) );
+
+		date_default_timezone_set( $timezone );
+		$this->assertEquals( 'http://example.org/2018/07/22/21/13/23', get_permalink( $post_id ) );
+	}
+}