Originally Posted by toms
Which is similar to many early methods of telling time: like the water clocks. All you need to know is the rate at which the water flows out of the jar and you can estimate reliably and repeatably how long it will take for a certain amount of water to flow out. Infact its similar to almost every measuring method we have that makes our world possible.
The only inacurracy that these have, actually, is that it's hard to know for sure if the element was made at the same time the rock formed; you'd have to know how much element was present in the rock at the beginning to really accurately measure the age. Something that is perhaps better for showing that such ages are possible is starlight and background radiation of the universe that the WMAP satellite measures. It's pretty clear that either things actually are that far away (and thus we can figure their minimum age by how far away they are), or someone's playing a rather big joke on us. There's no way we can know 'for sure,' so it's better to assume that it really is the truth - to do otherwise would be to deny science. Yeah, you can do it, but it doesn't get you very far.