Cutting it close?

I entered China on August 4th (this is the stamp that is on my passport) on a 90-day multiple entry visa. Since the 90 days are coming up, my company is sending me to Taiwan so that I can have another 90 days. My slight concern is, I'm flying out of Beijing on November 2, which as I counted, is exactly 90 days after August 4th.

I'm a little worried that this is cutting it too close, does the first day of the 90-day start on the first day that I arrive in China (so was day 1 for me August 4th, or August 5th?)?