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However, to draw this conclusion we have to assume that the rate of cratering has been the same in the past as it is now.And there are now good reasons for thinking that it might have been quite intense in the past, in which case the craters do not indicate an old age at all (see below).
In fact, it mentions an experiment in which DNA from a sample millions of years old was extracted successfully.
Ages of millions of years are all calculated by assuming the rates of change of processes in the past were the same as we observe today—called the principle of uniformitarianism.
If the age calculated from such assumptions disagrees with what they think the age should be, they conclude that their assumptions did not apply in this case, and adjust them accordingly.
Always the starting time of the ‘clock’ has to be assumed as well as the way in which the speed of the clock has varied over time.
Further, it has to be assumed that the clock was never disturbed.