'Retrieve the time stamp for when the certificate starts being valid
:return: A string giving the timestamp, in the format::
YYYYMMDDhhmmssZ
YYYYMMDDhhmmss+hhmm
YYYYMMDDhhmmss-hhmm
or None if there is no value set.'

def get_notBefore(self):

    return self._get_boundary_time(_lib.X509_get_notBefore)
