Example:
$timestamp = $bigQuery->timestamp(new \DateTime('2003-02-05 11:15:02.421827Z'));
public function testToString()
{
$value = new \DateTime();
$timestamp = new Timestamp($value);
$expected = $value->format('Y-m-d H:i:s.uP');
$this->assertEquals($expected, (string) $timestamp);
$this->assertEquals($expected, $timestamp->formatAsString());
}