The day you enter counts as your first day of your 30 days.You have 30 days,including that day,to stay in China.If you entered on the 13th of November,you will have used up 18 days up to and including the 30th of November.You must therefore exit before midnight Chinese time on the 12th of December, so as not to overstay your visa and face a fine of 500 Yuan/day of overstay,up to a maximum of 5000 Yuan if you uverstayed 10 days or longer.