Entering the time interval into the sheet and referring to that cell allows for more flexibility in the process. When someone wants to change the interval, you won't have to change the formula, just enter the new interval. You might think it won't change and you might think changing the one formula is no big deal, but that's because the example is simple. When applying these rounding formulas and functions to real apps, things usually get more difficult and less dependable. As sure as someone says "That'll never change!" -- it'll change.