So they tested some of the ring sequences by C14 to put the sequences in the 'right' order.
Some believe trees are known to be as old as 9,000 years. A lot of people doubt this claim for various good reasons I wont go into here.
You can find some further good information here: -- read the full page if you get the chance.
C) dating usually want to know about the radiometric dating methods that are claimed to give millions and billions of years—carbon dating can only give thousands of years.
Dates up to this point in history are well documented for C14 calibration.
For object over 4,000 years old the method becomes very unreliable for the following reason: Objects older then 4,000 years run into a problem in that there are few if any known artifacts to be used as the standard.