[SPOILERS] Don't read the following message if you don't want the show to be spoiked for you.
Itachi Uchiha killed his clan because he was ordered by Danzo and the Black Ops of the village to exterminate his clan. The hokage had heard rumors that the Uchiha clan wanted to take over the village and lead it on their own, so they had to be eliminated.
The dating of artifacts is done by a carbon dating age system based on
the development of tools and technology. They used the carbon that is left in
the artifacts of the deceased.
