Do anyone knows about the timings of buses that departs from dalian to beijing?

Well, you can take a bus to Beijing from DL Zhuanghe Passenger Transport Station.
The only direct long-distance bus departs at 11:28. And the trip takes 9-10 hours. The fare is CNY 261.5 per person.
Or, you may take a bullet train from Main Railway Station or North Railway Station and arrive at Beijing (about 6.5 hours).
You can choose the way you like.