I dread supporting O(hours). Clients often have wrong clocks. I've seen some client systems that enforced 'Not Before' and interpreted the datetime as local time and there were many users of that platform in the Americas; and my CA at the time insisted that Not Before was the time of issuance... lots of fun deciding how long to keep using a revoked certificate to balance the users with working revocation vs the users with broken Not Before checking. The client system I'm aware of is very dead, but maybe other systems managed similar.