dated to / dated at

Hello,

I am wondering which prep is more idiomatic to follow dated: to or at?

I have read such sentences as: The skull is dated at about 36,000 years old. But I also find such sentences as: we can date the skeleton to approximately 2500BC.

So, to and at are interchangable in those cases? In addition, can I use "to be" in a sentence, for example, the skull is dated to be about 36,000 years old?

Thanks in advance.