While developing my own IRC bot I noticed that the PING command does not get a proper reply from mIRC when sent from X-CHAT for example.
According to the CTCP spec The Client-To-Client Protocol (CTCP), mIRC should reply back the exact same message as the other party has sent to mIRC, but currently mIRC responds with a timestamp of its own current time (that is, current timestamp of when the CTCP request was received).
I hereby propose that the replying method would be changed accordingly.

It already replies exactly how you propose it should.

-
* Debug output on (@debug)
-
-> server PRIVMSG RoCk :
PING 1250515911

<- :RoCk!rock@host PRIVMSG RoCk :
PING 1250515911

-> server NOTICE RoCk :
PING 1250515911

-
[08:31] -> [RoCk] PING
-
[08:31] [RoCk PING]
-
[08:31] [RoCk PING reply]: 1sec
-
* Debug output off
-

Well, this doesn't seem like that to me:

<- :rX!~nnscript@99-499-24-720-ypm.kjj.estpak.ee PRIVMSG mikk36 :
PING 1250518200

-> zone.ircworld.org NOTICE rX :
PING 1250518180

I'm currently on three different networks and tested this on all three with the same results on each. It seems yours is being changed somewhere along the line. Possibly a script? nnscript?

I have to confirm Rock's findings on the 3 networks that I'm on.

One possibility that I can think of, other than a script, is that the network may not use standard protocols, but without the network information I have no way of confirming or denying this possibility.

Just realized that the information is in the OP's 2nd post.

From zone.ircworld.org

This confirms that the difference must be with a script being run by the OP, imo.

To echo the responses above, mIRC does in fact respond properly to CTCPs on its own. However as you probably know, mIRC can be extensively scripted, so you can and should expect any behaviour beyond the default from any mIRC user, and, frankly, any user on any IRC client.