I think what DeltaMac is getting at is that 9:30:50 is as good as 9:31 in human time for most things. Your wristwatch, computer, alarm clock, and microwave oven clock are all probably not synchronized to within 10 seconds of each other, so 9:30:50 on your computer (which is the clock the alarm clock program will use) may be 9:31:07 on your microwave, 9:29:32 on your wristwatch, and 9:35:03 on your alarm clock.
So, a timer going off on your computer at "computer time" 9:30:50 may not actually be 9:30:50 according to a bunch of other clocks. The significance of 9:30:50 is only relevant for anything else that specifically uses the same, exact clock (in other words, your computer's clock) -- it won't be good for websites if you're trying to time auctions (because no doubt eBay's auction computers are not synchronized to your computer clock), it won't be good for reminding you when the pizza is warm (because your stove clock isn't synchronized to your computer clock), and it won't be good for anything else that uses any other clock other than your computer's clock.
In other words, what is wrong with an alarm that chimes at 9:31, 9:32, 9:33, etc. instead of 10 seconds before each of those minutes? If you could explain in greater detail the need for this level of precision and synchronization between clocks, we may be able to help a little more.