Thanks for looking into it.

While confirming all the information you provided, I did another check, however the times now match and is displayed. This error happened right after we entered DST and mIRC was running during the transition. The timestamp 2020-10-03T17:16:15 seems to be 4:16AM (which is the correct time, so mIRC is adding DST maybe?)

Right, if it was only happening during the transition, this may mean that the times/DST in either mIRC or ZNC, or possibly the OSs on either machine, were not synchronized.

mIRC listens for a WM_TIMECHANGE message from Windows that lets it know when the time and/or DST state has changed, so the moment the DST change occurs, mIRC updates its $daylight value. If ZNC is not listening for DST changes (which seems unlikely), or the computers on which mIRC or ZNC are running are not updating at the same time or are using different DST transition rules, this would result in this issue. There are many reports online of Windows (including 10) behaving oddly during DST transitions.

At this point, without being able to reproduce it, its hard to tell what happened in this case.