The testing I did back then was done in thread https://forums.mirc.com/ubbthreads.php/topics/265145/gmt-number
where I was looking at the time-of-day being correct, and I did not test zzz at all.
That thread included https://forums.mirc.com/ubbthreads.php/topics/265267/re-gmt-number#Post265267
where Khaled mentioned fixing a DST bug related to zzz but didn't describe what that problem had been. Perhaps that fix had gotten reverted later in the thread when the TimeAPI changed.