It's not an assumption.
“The major glaring fault in the article is the false assumption that dinosaurs lived millions and millions of years ago”
Ha ha said to the above, “It’s not an assumption.”
Dating rocks by radioactive timekeepers is simple in theory, but almost all of the different methods (except for the isochron methods) rely on these few basic assumptions:
1. Beginning Conditions Known
2, Beginning Ratio of Daughter to Parent Isotope Known (zero date problem)
3. Constant Decay Rate
4, No Leaching or Addition of Parent or Daughter Isotopes
5. All Assumptions Valid for Billions of Years
6. There is also a difficulty in measuring precisely very small amounts of the various isotopes
Stanssen, Chris., Isochron Dating, Sept., 1998.
(http://www.talkorigins.org/faqs/isochron-dating.html)
Yes, actually, it is an assumption. You make the assumption that it rained last night when you see that the grass is wet. That may or may not be true. If true that does not make it something other than an assumption. An assumption is an assumption whether supported by facts or not.