Carbon 14 dating is calibrated using bristlecone pines, which in turn can be accurately dated by simply counting tree rings. In other words, you can count the rings in a series of trees (using overlapping rings series to bridge the multiple trees needed), then date a particular ring or group of rings. Thousands of samples dated in this manner allows accurate calibration of the carbon 14 method back to about 10,000 years ago.
Note that ice-core rings and varves and other tree-rings from other parts of the world (rather the Historical Edge of North America) also can be used. They all agree.