How to interpret time stamps in xt_recent?

I am trying to use the xt_recent netfilter under Linux 3.11 to implement a more reactive firewall. I want to be able to review when the most recent packet in any blocklist came in, but I don't know how to convert the last_seen and oldest_pkt fields into anything meaningful.

What exactly are these values representing? How can I convert them to a wall-clock time?