a question about duration of stay in China.

Hello, I have a business visa with 30 day duration of stay. I entered (via Shanghai Pudong) and went through customs around 6PM on May 17. Now, I am planning to leave (via Beijing) on June 16. My flight is scheduled to leave at 7:10PM, but I plan on going through the customs around 4PM.

Am I overstaying? Does the duration-of-stay clock start when entering customs and end when exiting customs? Is 24-hr stay considered one-day stay?