I have a 6 month, multiple entry visa, valid for 30 days each visit, that expires on March 18th.

I last entered China on March 4th. Do I have to leave China on March 17th? Or can I leave on th 18th, to apply for a new visa? Or, can I stay 30 days from March 4th? How does this work? I do not want to 'overstay' my visa.