In order to optimize network traffic and reduce the amount of checks that need to be done by a client or server to validate an application certificate signed by a CA, it would be beneficial to return a response with a validity period or the next Update time.
So the client/server can reduce the number of checks to a new certificate and cache the response until the next update is due.
The validity time response may also be 'unknown' for cases where an proper validity time can not be determined, this case should be specificed (e.g. DateTime.MinValue),
The update time would also allow for the implementation option to offload the validity checks to a time which is not so critical compared to the check during the session connection.

For the OCSP protocol and for CRL there is information included to calculate the value (RFC6960):

The thisUpdate and nextUpdate fields define a recommended validity
interval. This interval corresponds to the {thisUpdate, nextUpdate}
interval in CRLs. Responses whose nextUpdate value is earlier than
the local system time value SHOULD be considered unreliable.
Responses whose thisUpdate time is later than the local system time
SHOULD be considered unreliable.