public function test_build_mysql_datetime_with_relative_date() {
    update_option( 'timezone_string', 'Europe/Kiev' );

    $q = new WP_Date_Query( array() );

    $yesterday = new DateTimeImmutable( '-1 day', wp_timezone() );
    $expected  = $yesterday->format( 'Y-m-d H:i:s' );
    $found     = $q->build_mysql_datetime( '-1 day' );

    $message = "Expected {$expected}, got {$found}";
    $this->assertEquals( strtotime( $expected ), strtotime( $found ), $message, 10 );
  }