I noticed this tonight when Daylight Savings kicked in at 2am (jumped from 1:59am to 3:00am).
mIRC (6.12) doesn't seem to want to believe the time that Windows is telling it.
For instance, the time is currently 5:22am according to the Windows clock, however if I type a message in mIRC, it shows up as 4:22am. I can change this by changing the Windows clock.
However, changing the Windows clock to a time prior to DST kicking in results in a 'correct' timestamp being shown, but if I change the time to (say) 2:23am, mIRC will display a 1:23am timestamp. If I change the Windows time to 1:23, then mIRC will display a 1:23am timestamp.
Something weird is happening here - I haven't tried restarting yet though.