According to the help file, from /help $ctime
Returns total number of seconds elapsed since 00:00:00 GMT January 1, 1970 based on your system time
Since $ctime won't return a negative value (something that I know I've seen mentioned on these boards) then it makes sense that a date with year before January 1 1970 is not going to return a correct response. The fact that you went so far back in time that you found dates that return nothing rather than an error is interesting, but, in my opinion, not very relevant.
As to the dates in the future, ie: 2038, I would imagine that the coding for mIRC will be updated to accomodate those dates as we get closer to them. Right now, by the calendar that I use, that's still over 3 decades away.