I need to know when an SSL certificate actually expires. Does it just look at the expiry date, or does it also take into account the expiry time?

Let's say a certificate expired on 1/1/2013 at 11am. Does that certificate expire at 11:01am or is it only the following day that the certificate expires? I have been told both are true.

Hope that makes sense! Our suppliers messed up and let our certificate expire, and I'm trying to figure out how much compensation we're owed.

I found this question Details on exact expiration datetime of an SSL certificate? but it didn't quite answer what I need (and I didn't like to revive a dead question).