[Aule]

Since there's a server restart in the morning I'll be able to test this.

In my addon init function I implemented this:

-- Has there been a server reboot?
local current = GetTime()
if (GrudgeBook_Config.LastInitTimestamp < current) then
    -- No, server is still up since last run.
else
    -- Yes.  Invalidate all timestamps.
    GrudgeBook_InvalidateLastSeen()
end
GrudgeBook_Config.LastInitTimestamp = current

For the rest of the mod if the timestamp is nil, I display it as "Unknown".

Seconds since epoch on the server would sure be nice right about now!!