How do we count the no. of days to qualify for Visa exemption for Singapore Passport holder?

If I leave Singapore to Xiamen on 1 Dec 2012 ,

and come back from Xiamen to Singapore on 16 Dec 2012.

Do I need to apply for a Visa in this case?

When is the latest date I must leave China if I do not apply for one?

I am not sure how the period of "up to 15 days" is counted, is it 16-1=15 ?? Please enlighten me. Thank you.