Actually, Tim, they do have several accurate methods. There are relative dating methonds (Stratigraphy, flourine dating, biostratigraphy, poleomagnetic reversals) and chronometric dating methods (carbon-14 dating, potassium-argon dating, argon-argon dating, etc.) I don't know where you got the idea that they just pick numbers out of thin air, but maybe you should do a little research.
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