Third, some creationists like the members of the RATE group theorize there was a pulse of accelerated radioactive decay around the time of the Flood, but this would not apply to the post-Flood era.Fourth, while it is true that we cannot know the past (this is the great limitation of experimental science), it is sometimes convenient to use the opposition's numbers against them.
Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of 415 to 420 million years.
However, I will stand by my statement with this defense: First, we do not need changing decay rates to explain 14C dating.
There are enough uncertainties in the physical history of earth to throw great uncertainty on the early dates.
There are two main methods determining a fossils age, relative dating and absolute dating.
Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.