if you didnt have the timestamp on, how could mIRC apply the timestamp retroactively unless mIRC was keeping a second log of everything that happened that did have timestamps on?
what you might think of to get what you want is a @custom window that you set up on events and script it to timestamp certain things like say on *:open: and with the time who the nick is the address or whatever you want.
Then all you need do is open that window and find the relative event and check the time
