def get_notBefore(self):
return self._get_boundary_time(_lib.X509_get_notBefore)<EOL>
Get the timestamp at which the certificate starts being valid.

The timestamp is formatted as an ASN.1 TIME::

    YYYYMMDDhhmmssZ

:return: A timestamp string, or ``None`` if there is none.
:rtype: bytes or NoneType