Since that time, CALIB, now renamed Int Cal, has been refined several times--as of this writing (January 2017), the program is now called Int Cal13.
So, if you measure the amount of C14 in a dead organism, you can figure out how long ago it stopped exchanging carbon with its atmosphere.
Trees maintain carbon 14 equilibrium in their growth rings—and trees produce a ring for every year they are alive.
Although we don't have any 50,000-year-old trees, we do have overlapping tree ring sets back to 12,594 years.
We should see some interesting results in the very near future.
Carbon-14 dating can be used on objects ranging from a few hundred years old to 50,000 years old.