For older periods we are able to use other records of with idependent age control to tell us about how radiocarbon changed in the past.
It is calculated on the assumption that the atmospheric radiocarbon concentration has always been the same as it was in 1950 and that the half-life of radiocarbon is 5568 years.
For this purpose `present' refers to 1950 so you do not have to know the year in which the measurement was made.
Using very old trees (such as the Bristlecone Pines in the western U. A.), it is possible to make measurements back to a few thousand years ago.
To extend this method further we must use the fact that tree ring widths vary from year to year with changing weather patterns.
By using these widths, it is possible to compare the tree rings in a dead tree to those in a tree that is still growing in the same region.
To give an example if a sample is found to have a radiocarbon concentration exactly half of that for material which was modern in 1950 the radiocarbon measurement would be reported as 5568 BP.
For two important reasons, this does not mean that the sample comes from 3619 BC: Many types of tree reliably lay down one tree ring every year.