mIRC also has local output that may be timestamped, for example from local commands.
This alone makes me wonder how mIRC's automatic parsing of remote timestamps could ever be effective. It would be extremely confusing if an unscripted line of text came in with timestamp X and then a local command echo'd data with a completely different timestamp Y. It would seem extremely jarring and buggy to any user, I would imagine:
(17:53) <nickname> hello world
(12:01) * database backup complete
(17:54) <nickname2> hey nick, how are you?
If this happened, you'd probably start seeing a lot more invalid bug reports being posted about mIRC improperly handling timestamps, not unlike
this one, in fact. The problem is, it's not a bug, it's a complete misappropriation of timestamps.