Buying Jiuzhaigou bus ticket in advance?

I intend to take a bus from Chengdu Xinnanmen bus station to JZG. If I buy the ticket one day before I leave, would I be able to get the ticket? Pl advise if I need to buy the bus tickets few days in advance. I am going to JZG on 21 May 2014. What time does the bus ticket office open?