PZ Myers gave a talk to the Freethinkers group at St. Olaf the other day. I wish I had been there.
He has the transcript up and I think you should read it.
I am here to bring you some good news.
The universe is about 13.7 billion years old, plus or minus a few hundred million, and the earth itself is about 4.6 billion years old. How do we know this? The work of astronomers in measuring cosmological constants, in calculating the age of stars and the size of our universe; the work of physicists on principles of radioactive decay, and measurements of the age of rocks; the work of geologists in charting the many layers of rock and puzzling out the mechanisms of change.