Thanks for your bug report. I have not been able to reproduce this issue so far. Just to confirm:
1) You connect to a server through ZNC.
2) Your computer says the time is "04:16" and you are in DST (+1 hour).
3) You send a private message to your own nickname.
4) The private message comes back to you ZNC prefixed with "@time=2020-10-03T17:16:15.448Z".
5) mIRC converts this to local time and reports "05:16".
I installed ZNC here locally and followed the above steps:
1) Connect to a server through ZNC.
2) Computer says time is "10:35" in DST (+1 hour).
3) Send private message to my nickname.
4) Private message comes back ZNC prefixed with "@time=2020-10-06T09:35:38.843Z"
5) mIRC converts this to local time and reports "10:35".
So in my case, mIRC is correctly adjusting the UTC time stamp to local time and adjusting for DST.
I also ran mIRC and ZNC on both Windows and a VirtualBox, connecting in both directions, changing the timezone to locations with and without DST, and the resulting time was always correct.
In which timezone is your ZNC located? Can you confirm that the PRIVMSG timestamp it is sending is the correct UTC?