Hello all,Visa duration

I am aware that this questions has been asked a couple of times, but does the 30 days entry permit duration start counting from the day FOLLOWING the entrance day in China? Has anybody ever tried this personally? The information on the web is very confusing. I have entered China on the 15th of May, when is the last permitted day of stay? 14th or 13th of June? Thank you so much for your help!