ユニットテストで日付指定で作った投稿の日付の取得

以下のテストがある際に、get_the_date() で日付を取得すると、January 1, 1970が返ってきます。

<?php

class SampleTest extends WP_UnitTestCase {

	public $the_post;

	public function setUp() {

		parent::setUp();

		$post_date     = '2015-09-21 00:00:00';
		$post_date_gmt = get_gmt_from_date( $post_date );

		$this->the_post = $this->factory->post->create_and_get( array(
			'post_title'    => 'Posted on 21th Sept, 2015',
			'post_date'     => $post_date,
			'post_date_gmt' => $post_date_gmt,
		) );

		update_option( 'date_format', 'F j, Y' );

	}

	public function test_this_test() {

		// 通る
		$this->assertEquals( 'Posted on 21th Sept, 2015', get_the_title( $this->the_post ) );

	}

	public function test_date_format_none() {

		$this->assertEquals( 'September 21, 2015', get_the_date( '', $this->the_post ) );

	}

}

$ phpunit
Installing...
Running as single site... To run multisite, use -c tests/phpunit/multisite.xml
Not running ajax tests. To execute these, use --group ajax.
Not running ms-files tests. To execute these, use --group ms-files.
Not running external-http tests. To execute these, use --group external-http.
PHPUnit 4.3.1 by Sebastian Bergmann.

Configuration read from /var/www/wordpress/wp-content/plugins/buddha-calendar-converter/phpunit.xml

.F

Time: 1.75 seconds, Memory: 15.25Mb

There was 1 failure:

1) SampleTest::test_date_format_none
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'September 21, 2015'
+'January 1, 1970'

/var/www/wordpress/wp-content/plugins/buddha-calendar-converter/tests/test-sample.php:34

FAILURES!
Tests: 2, Assertions: 2, Failures: 1.

• WP_Postオブジェクトをvar_dumpすると日付がきちんと入っている
• post_date_gmtパラメータの有無は結果に関係がない
• post_dateパラメータの有無も結果に関係がない
• $this->go_to( ‘/?p=’ . $this->the_post->ID ); してから get_the_date(); しても同じ

という状態です。