This implies the earth is at least 20 million years old.
Astronomical cycles can also be used to measure relative age.
In other words, half of the radioactive isotope in a sample would have decayed to Nitrogen-14 (N-14) in just 5,730 years.
C-14 dating of carbon-bearing materials is therefore limited to roughly 50,000 years.
Some claim Genesis in particular, and the Bible in general looks mythical from this standpoint.