and this

"Now, this technique is pretty cool if you want to calculate the age of a tree, for example, or even an Ancient Egyptian king a few thousand years young. But in the case of millions of years old dinosaur bones, any 14C present would have long since decayed, making radiocarbon dating only good for about samples that are at most 75 000 years old.

Accurate dating of samples requires that the parent radioactive isotope has a long enough half-life such that it will still be quantifiable today. So, what are our options?

Uranium-Lead (U-Pb) dating is the go to method for dinosaur age samples, being able to accurately date rocks (yes, rocks, we’ll come to that later) between 1 million and 4.5 billion years old.

Two decay chains are measured in this type of radiometric dating, 238U to 206Pb and 235U to 207Pb, via a cascade of alpha and beta decay with half-lives of around 4.5 billion years and 700 million years, respectively."

Just curious how they calculated a half life of 4.5 billion years How do they determine what it started at ? .