One Thing I Don't Understand - Validity (Enter Before) VS. Entry

So, I have 6 month double-entry visa. It says: "Enter Before 20 DECEMBER, 2017". I can stay for 90 days after each entry. For example, I used my first 90 days and left China peacefully. Then I decided to re-enter China second time on 20 NOVEMBER, 2017 (1 month left before expiration).

So, when I enter China second time, will I have to stay for only 1 month? (before 20 December, 2017) Or, can I still stay for 90 days? (till 20 February, 2018 - which is AFTER validity date).