SAML2\Assertion::getNotBefore PHP Method

    public function getNotBefore()
    {
        return $this->notBefore;
    }

 public function validate(Assertion $assertion, Result $result)
 {
     $notBeforeTimestamp = $assertion->getNotBefore();
     if ($notBeforeTimestamp && $notBeforeTimestamp > Temporal::getTime() + 60) {
         $result->addError('Received an assertion that is valid in the future. Check clock synchronization on IdP and SP.');
     }
 }