Why should you not restore a DC that was backed up 6 months ago?

Why should you not restore a DC that was backed up 6 months ago?

As I am learning Active Directory Domain Services I came across this question in one of the blogs but I was unable to find a detailed answer. So please can anybody explain this concept to me.

There is a thing called tombstone lifetime in Active Directory. When you delete an object in Active Directory it is not immediately gone, it is converted to a tombstone and this information is replicated to the other DCs. When the tombstone lifetime is reached the object will be purged. If you restore prior to a state before the deletion and the tomsbtone is not replicated to the restored DC before it expires, the object will remain present in your restored DC but not in the other DCs. Now you have inconsistent data. Default tomsbtone lifetime for Server 2008 and onwards is 180 days (= 6 months).