I have just tested mIRC v7.34 with the latest version of ZNC v1.5 for Windows
and the timestamps are being displayed correctly here. When I enable debugging with /debug on, I can see mIRC enabling CAP znc.in/server-time-iso and from that point onwards it displays the per message timestamp that is sent by ZNC in the @time= prefix.
Are you using any scripts that may be modifying the output? If you install a clean copy of mIRC, with no scripts, using the "portable" option in the installer, into your Windows Documents folder and run mIRC from there, do you see the same issue?