Add human-readable expiration convertion in compatibility.htm

[martgil]

When reviewing for keys' expiry date, it is still in epoch time format. While its okay to have in the current build, it doesn't seemed to be so reliable when doing quick troubleshooting then we will still need to convert that to human readable date/time.

[martgil]

Perhaps, date/time format such as the one used for the Sig creation time will do. Just for debuggers and regular users not to take extra effort to convert these epoch time to readable date/time.