Khaled, I don't remember if this is the same as the previous timestamp issue, I don't use a ZNC regularly but I test from time to time.
mIRC shows the timestamp incorrectly for the playback buffer, it looks like that all the timestamps are adjusted to the CONNECTION time and not to the time where the MSG was sent, I've tested other clients and web ones and they work fine.
I saw on the ZNC webpage this msg "If you have a modern client which supports server-time capability, timestamps are automatically shown correctly. Otherwise, one of scripts below might help. But in that case consider upgrading your client instead.", does mIRC support server-time?
Not sure what's going on here, if you can double check. Thank you.