Jose,
I believe that I explain this as part of the lesson of The PM Podcast. Both formulas are correct. But there is a difference in "philosophy" behind them.
* If you follow my formulas (with the '-1'), then you assume that your project begins on "Day 1"
* If you follow the other formulas, then you assume that your project begins on "Day 0"
I prefer my formulas for 2 reasons:
* If your boss tells you that your project begins on the first day of the next month, then he means "January 1" and not "January 0"
* All modern scheduling tools by default start on day 1 and not on day 0 (mainly because they are calendar based)
That's really all there is to it.