These two measures of time will only be the same if all of the assumptions which go into the conventional radiocarbon dating technique are valid.
It is not correct to state or imply from this evidence that the radiocarbon dating technique is thus shown to be generally invalid.
The problem with freshwater clams arises because these organisms derive the carbon atoms which they use to build their shells from the water in their environment.
Long tree-ring chronologies are rare (there are only two that I am aware of which are of sufficient length to be of interest to radiocarbon) and difficult to construct.
They have been slowly built up by matching ring patterns between trees of different ages, both living and dead, from a given locality.
MYTH #2 Radiocarbon dating has established the date of some organic materials (e.g., some peat deposits) to be well in excess of 50,000 years, thus rendering a recent creation (6 to 10 thousand years ago) impossible.
Some organic materials do give radiocarbon ages in excess of 50,000 "radiocarbon years." However, it is important to distinguish between "radiocarbon years" and calendar years.
First, any instrument which is built to measure radiocarbon has a limit beyond which it cannot separate the signal due to radiocarbon in the sample from the signal due to background processes within the measuring apparatus.
Even a hypothetical sample containing absolutely no radiocarbon will register counts in a radiocarbon counter because of background signals within the counter.
It is not difficult to see how such a claim could arise, however.
There are two characteristics of the instrumental measurement of radiocarbon which, if the lay observer is unaware, could easily lead to such an idea.
Some may have mistaken this to mean that the sample had been dated to 20,000 radiocarbon years.