def METHOD_NAME(cert):
    """Verify that the supplied client cert is not invalid."""
    now = datetime.utcnow()
    if cert.not_valid_after < now or cert.not_valid_before > now:
        raise ValueError('Client certificate expired: Not After: '
                         '{:%Y-%m-%d %H:%M:%SZ} '
                         'Not Before: {:%Y-%m-%d %H:%M:%SZ}'
                         .format(cert.not_valid_after, cert.not_valid_before))