Obviously, you are absolutely right. If as accurate as possible timekeeping is of importance to you, and for some weird reason you are limited to using hours:minute clock (for example, the clock is integrated part of some control device, and their designers didn't anticipate the importance of high resolution time, hence did not support setting/displaying seconds), you will need to set the clock to show 12:00 at 11:59:30. This way, you halve the absolute error. Nothing special here, it's the usual rounding rule; nearest neighbor instead of floor function.
But usually, you just choose to have enough resolution so that you can ignore rounding. Even in math where you are usually "supposed" to round results of calculations, this is wasted effort because the effect is so small (and if it matters, you should have added one digit anyway). If you go to the lengths of looking at the next digit and making a decision based on it, you could just write it down.