Methods for dating the earth
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).Long-age proponents will dismiss this sort of evidence for a young earth by arguing that the assumptions about the past do not apply in these cases.In other words, age is not really a matter of scientific observation but an argument about our assumptions about the unobserved past.
The reference to the "way[s] in which the speed of the clock has varied over time" are a very thinly veiled attack on a bedrock assumption of scientific practice, uniformitarianism, in (for the sake of argument) contradistinction to catastrophism.
Errors tend to be random; for the estimate to be incorrect, the errors would have to be the same for all samples and all methods, which is extremely unlikely.
A single observation of a wildly discordant estimate is not enough to overturn the concordant estimates, because observations are always subject to errors and outliers.
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.