'This extension indicates the suspected date/time the private key was
compromised or the certificate became invalid. This would usually be
before the revocation date, which is when the CA processed the
revocation.
:return:
None or a GeneralizedTime object'

@property
def invalidity_date_value(self):

    if (self._processed_extensions is False):
        self._set_extensions()
    return self._invalidity_date_value
