Untrue. In fact C-14 dating is now routinely calibrated using dendrochronology.
The Grand Canyon is supposed to be the result of that little river running through it for ages, and yet, its entry point is about 3,000 feet below the highest point.
So there was geological uplift of a mountain range in the interim. What's the problem?
The speed of light is seen as the same (light years is a distance, not a speed) and yet it has been controlled, slowed down, sped up in a lab, and we know that it has naturally been slowing down over the years.
The speed of light in the vacuum has never been altered. Its apparent speed in a medium can be reduced, but not (AFAIK) increased. And claims it has changed over time do not stand up to careful scrutiny.
Please fill in some detail of your idea for me.