Transportation from Zhangjiajie city to park entrance.

Me and my friend will arrive in ZJJ in the beginning of next week. I've learnt that there is a bus connecting ZJJ city with the main entrance to the national park in Wulingyuan. Is there any information available on how frequently these buses depart? Also, it would be great to know at about what time the first departure is from ZJJ and the last one back to ZJJ from the park. Thank you in advance!