@deftypefun {int} {gnutls_x509_crt_get_private_key_usage_period} (gnutls_x509_crt_t @var{cert}, time_t * @var{activation}, time_t * @var{expiration}, unsigned int * @var{critical})
@var{cert}: should contain a @code{gnutls_x509_crt_t}  type

@var{activation}: The activation time

@var{expiration}: The expiration time

@var{critical}: the extension status

This function will return the expiration and activation
times of the private key of the certificate. It relies on
the PKIX extension 2.5.29.16 being present.

@strong{Returns:} On success, @code{GNUTLS_E_SUCCESS}  (0) is returned, @code{GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE} 
if the extension is not present, otherwise a negative error value.
@end deftypefun