the actual expiring date for my s2 visa when i enter mainland 13th Dec , 2013

Please I'd be very grateful if you can help me to know the actual expiring date of my S2 which was issued on 1st November,2013 and I've planned to enter china on December 13th, 2013 (2013/12/13). I want to know the 180 days is counted from the day it was issued or it will be counted from the first day I'll enter mainland.
thnks