'Sets the certificate activation time.'

def not_valid_before(self, time):

    if (not isinstance(time, datetime.datetime)):
        raise TypeError('Expecting    datetime    object.')
    if (self._not_valid_before is not None):
        raise ValueError('The    not    valid    before    may    only    be    set    once.')
    if (time <= _UNIX_EPOCH):
        raise ValueError('The    not    valid    before    date    must    be    after    the    unix    epoch    (1970    January    1).')
    if ((self._not_valid_after is not None) and (time > self._not_valid_after)):
        raise ValueError('The    not    valid    before    date    must    be    before    the    not    valid    after    date.')
    return CertificateBuilder(self._issuer_name, self._subject_name, self._public_key, self._serial_number, time, self._not_valid_after, self._extensions)
