public function test_validate_date_values_should_process_array_value_for_day() {
    $p1 = self::factory()->post->create( array( 'post_date' => '2015-01-12 00:00:00' ) );
    $p2 = self::factory()->post->create( array( 'post_date' => '2015-01-10 00:00:00' ) );

    $q = new WP_Query(
      array(
        'date_query' => array(
          array(
            'compare' => 'BETWEEN',
            'day'     => array( 9, 11 ),
          ),
        ),
        'fields'     => 'ids',
      )
    );

    $this->assertEquals( array( $p2 ), $q->posts );
  }