I'll start off by saying that I'm not sure if this is related to this recent post or not:http://forums.mirc.com/ubbthreads.php/topics/248878/7.36_Timestamp_not_correct
In any case, I have had this problem for years now. I'm in Detroit, MI, USA, and whenever DST starts or ends, while the actual time (i.e. in HH:nn:ss format) is correctly updated in my timestamps, the UTC offset is not.
For example, DST just ended here this last Sunday, November 2, 2014, and my UTC offset should be -5 instead of -4. But $asctime(z) returns -4, $daylight returns 3600, $logstamp and $timestamp both show a -4 where the z in the format is and $timezone returns 14400 (which is 4 hours).
So far, the only fix for this has been to restart mIRC entirely, which makes it pull the correct timezone offset, but it doesn't appear to do it on its own if I leave it running during the DST switch. I haven't left it running for extended periods of time after the DST switch to see if it does correct itself eventually, but I have left it running currently so I could get the above results.