visa expiration

I have L VISA , 2 entries,date of issue is 04april2012. I already enter once last month and I want to enter the country again. It said, enter before 4th of July. My question is, can I enter before 4th of July and stay in China for 30 days? I want to stay there from 3rd of July until 3rd of august. Or, do I need to enter the country asap then leave the country before 4th of July?
Waiting to hear from you..thank you.