Date/Time: Remove `mysql2date()` usage in `get_feed_build_date()` to ensure the output includes correct timezone offset.

With the changes in [45908], `mysql2date()` works correctly for all local time inputs, but should not be used for UTC time inputs.

Add a unit test.

Props Rarst, lisota.
Fixes #48675.

git-svn-id: https://develop.svn.wordpress.org/trunk@46756 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/feed.php

@@ -682,7 +682,13 @@ function get_feed_build_date( $format ) {
 	}
 
 	// Determine the maximum modified time.
-	$max_modified_time = mysql2date( $format, max( $modified_times ), false );
+	$datetime = date_create_immutable_from_format(
+		'Y-m-d h:i:s',
+		max( $modified_times ),
+		new DateTimeZone( 'UTC' )
+	);
+
+	$max_modified_time = $datetime->format( $format );
 
 	/**
 	 * Filters the date the last post or comment in the query was modified.

tests/phpunit/tests/date/getFeedBuildDate.php

@@ -0,0 +1,39 @@
+<?php
+
+/**
+ * @group date
+ * @group datetime
+ */
+class Tests_Date_Get_Feed_Build_Date extends WP_UnitTestCase {
+
+	function tearDown() {
+		global $wp_query;
+
+		update_option( 'timezone_string', 'UTC' );
+
+		unset( $wp_query );
+
+		parent::tearDown();
+	}
+
+	/**
+	 * @ticket 48675
+	 */
+	public function test_should_return_correct_feed_build_date() {
+		global $wp_query;
+
+		$timezone = 'America/Chicago';
+		update_option( 'timezone_string', $timezone );
+
+		$post_id = self::factory()->post->create(
+			array(
+				'post_date'     => '2018-07-22 21:13:23',
+				'post_date_gmt' => '2018-07-23 03:13:23',
+			)
+		);
+
+		$wp_query = new WP_Query( array( 'p' => $post_id ) );
+
+		$this->assertEquals( '2018-07-23T03:13:23+00:00', get_feed_build_date( DATE_RFC3339 ) );
+	}
+}