Further cross-check assumptions such as the constant rate of atomic decay and constant lightspeed by observing atomic decay rates in distant supernovae.
However looking back into the last several billion years we see everything looking exactly as it would if the speed of light were the same then as it is now. We make predictions about how things would look if lightspeed were constant, and those predictions are borne out. This process is known colloquially as "doing science".
Only when we go back almost to the dawn of time, more than 10 billion years ago, do we start to see apparent effects which *might*, *tentatively*, be explained by the speed of light being different then from its current constant value. As your original question referred to the calibration of events that occurred less than 100 million years ago I have to wonder if you actually read the articles that you posted links to. There is no comfort for YEC positions in them, and no comfort for those who'd question the assumption that at least over the last 4.5 billion years (the approximate age of the earth, calculated using different methods to arrive at answers within 1% of each other) lightspeed and atomic decay has been constant.