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Dr. Gerald Schroeder is not a geologist, but he has a lot to say about the age of the universe based on Einsteins theory of relativity. If you want to look at decay rate you have to know that time itself is relative. This is creationism and actually this is Einsteins theory of relativity that has pretty much been demonstrated to be true over and over again.
Time is relative to how we measure it which is based on the length of a year, which is the time it takes for the earth to make a complete orbit around the sun. That is a year and we measure time by divisions of that standard. So enough about what is relative. To conflate what time is beyond that standard is off topic.
As for decay rates we know they have not changed. We know this on three levels.
1. If there were some physical change in the physical structure and properties of any element those changes would be seen at the point of change. Conversely, in dating rocks through numerous different methods and with numerous different isotopes, we so no change in those physical properties at any point in time what so ever. Measuring radionuclide decay rates is a continuous ongoing endeavor in labs around the world, not to see if a decay rate will change, but to measure those rates more accurately.
2. Supernovae produce a large quantity of radioactive isotopes from gamma ray with frequencies and fading rates that match present decay rates. This includes supernova 1987A, 169,0000 light years distant, SN1991T, sixty million light-years distant, and observations of supernovae several billion light-years away. (Knödlseder 2000), (Prantzos 1999), (Perlmutter et al. 1998).
3. And a special note, do not confuse the well known and studied oscillations of some cosmogenic istopes due to Earth's position in its orbit around the sun. This does not affect any dating method. Oscillations are just that, oscillations, not rate changes, because they are consistent.