Science uses many dating methods to determine the age of the earth, of the layers in the earth, and of fossils. We will discuss some of those on this page.
Kent Hovind has made himself wealthy (and wound up in jail for tax evasion and "structuring cash transactions") by claiming that "there is only one place in all the world to see the standard geologic column. That's in the textbook!" (I took that quote from WasDarwinRight.com, and he got it from Dan Janzen's Revolution Against Evolution site, but Kent Hovind made it famous.)
The fact is, the geologic column is found many places around the earth. Note on that page that drilling led to one more place being found in 2003.
Mr. Hovind is also fond of saying that geologists use fossils to date the layers of the geologic column, but they also use the layers of the geologic column to date the fossils. Amazingly, this claim of Mr. Hovind's is true; however, there's a good reason why.
There are also some amazing consistencies in the geologic column that cannot be explained except by the fact that our dating methods are accurate. For example, why do we think an asteroid impact caused the mass extinction of the dinosaurs?
There are many forms of radioactive (or radiometric) dating methods. Potassium-Argon and Uranium-Lead (of which there are two forms) date rocks to time frames of hundreds of millions of years. Carbon-14 on the other hand dates formerly living organisms to thousands of years, maximizing at around 50,000 years. There are a number of other radioisotopes that can be used to date geological formations as well, and often multiple methods can be used to cross-check each other.
There's lots more! Stay tuned! There's the light from stars, simply counting successive layers of earth, and more.