it needs to take in two very rare occurrences into consideration -- daylight savings time change, and system clock adjustments.
That's just one thing: system clock adjustments. But you are over thinking it, you shouldn't be taking this into account, if you are afraid it won't work if the system clock is changed then don't be, it's ridiculous to take that event into account, at best you would have a function triggering when the system clock is changed, and you would readjust your timer there.
That said, your claim are wrong, /timer -h HH:nn is as accurate as you can get, using a timer to get more accuracy because you think the timer itself isn't accurate is.. well, contradictory.
What are you asking for exactly? Using -h is basically your idea of calling an extra timer to check more often if the time elapsed, however this is not done with msl and it's not much more often than your nine seconds.
Assuming mIRC is busy, there is nothing you can do to get your timer to trigger at the specified time, it will always trigger later.
Assuming mIRC isn't busy, a multimedia timer (-h) should trigger with the specified time.
In short, if what you are asking for is a way to get more accurate timer, it already exists and it's using -h, you should use it.
