Can i enter into china jus before the day of expiration of my residence permit?

Hi, I'm CCS working on a z-visa and my residence permit expires on Dec 20th. I need to go to another country for a month, probably Nov 1st to Nov 30th. Can i enter China on Dec 1st? (or can i enter china on Dec 17th?) Another question is, when should i extend my residence permit? Should a apply for extension just before 1 week or 2 weeks or..from date of expiry? or can i a apply for extension now (in october) and then visit another country?