Which is the limit day calculated for the visa duration?

Hi,

I have a F-Visa with "60 days after entry".

Which is the limit day in which I have to take a flight out of China?

For simplification, let's consider that my visa was "5 days after entry".

If I entered to China the 14th of September, how is the limit day calculated?

Option A)- From the 14th to the 15th, it makes one day; From the 15th fo the 16th, it makes the second day.... and so son --> Limit day to leave the country would be the 19th of September

Option B) 14th = 1st day; 15th = 2nd day; 16th = 3rd day.... --> Limit day to leave the country would be the 18th of September

Thanks in advance!

Best Regards,

Javier