I think $uptime($server,1) is what you want (connection time to the server) .. $uptime(mirc,1) returns the amount of time you have had mIRC open (connected or not).
In any case, create yourself an alias and put that alias name in your Alt-O -> IRC -> Messages -> Quit message: $quitMessage
alias quitMessage return Connected to server: $uptime(server,1) $+ , mIRC Open: $uptime(mirc,1) $+ , Time since reboot: $uptime(system,1)