What if the entry to China is within 30 days before the visa expiring?

I am going to China and have a Chinese entry permit which expires mid way through April. My trip will be from mid to end March. Does there need to be still 30 days validity on the entry permit after I leave China? In reality there will be 14 days between end of trip and end of entry permit. I am wondering if there are rules like having 6 months validity on one's passport?