Validity versus duration of stay

I am landing in China on Aug 17th and leaving on Sept 12th. I am applying for a single entry 30 day visa. It is valid from 3 months of being issued. Although I will land there within 3 months, I will be over the 3 months the during the latter half of my stay. Does the 3 month validity only apply to landing there. Then once I land within the 3 months does my 30 duration of stay kick in.