Fossils are dated by the strata they are found in. Wanna guess how the strata is dated?
Actually this is another popular mistake promoted by creationists. The recognition that different rock layers, strata, were deposited at different times was made by John Hutton in the late 1700s. His most important discovery, Hutton's Unconformity, was made in 1787.
This greatly influenced Charles Lyell who is famous for his series of books, Principles of Geology, published between 1830–33. Lyell realized that the simple organization of rock strata, one on top of another could be extended by Hutton's proof of great time between depositions. By combining mineral, chemical and fossil data, strata not in direct contact could be placed into a proper sequence. This was the origin of the "index fossil" idea.
Since radioactivity was not discovered until the late 1800s, and atomic isotopes not until 1913, there was no way to accurately date any of the geological ages. What geologists did have was a well established sequence of events, and that was about all. It was not until the "Atomic Age" starting in the 1940s that we could reasonably measure the amount of radioactive isotopes in a rock, and use that data to calculate the age of the sample. It took another 30 years before the many methods were standardized, and calibrated. This made direct dating practical for the first time.
So today we see in publications a Ur/Pb, or Ar/Ar date from a volcanic ash layer, or a magnetic polarity stage as giving a direct dating of the deposit. But, what if there is no appropriate ash layer? Then you will see a "biostratagraphic" date based on the nearest location to the study project that had 1) directly dated material, and 2) the same fossils. It isn't as good, but we cannot edit mountains the way people change books.
This is where the creationists twist the facts- the direct age of the fossils is always determined by independent physical methods.