Duration of stay is counted from the day of arrival or from the next day of arrival.

D/S,I am an Indian national working here in nanjing. My wife & daughter came to china for holidays & their return ticket to India is on 28th May. They arrived in China on 28th April & their Visa stamped in passport for 30 days. I want to know whether the duration of stay is counted from entry date or from the next day after entry. Because my wife & daughter has to travel back on 28th May for which the return ticket is already confirmed.
So the days of stay considered from 28th April or from 29th April please help me to explain.

Regards,

Rajanna K V
Ph-15151869451