How long ago did something in the past happen, or how old is this piece of stuff?  These are often very difficult questions to answers.  "Dating" something is a fuzzy science that depends on decay rates and dates for things we can measure today, and then extrapolating beyond what we are sure of into the unknown past, assuming decay rates and such were the same back then.  An intriguing aspect of this science is that there are often many alternative ways to date something.  Often a scientist will pick that method that he feels best agrees with what he feels the answer should be, vs. going with the best method for the situation.  Check out the following links to get an insight into this field of science.  They will help arm yourself with a discerning wisdom about something that is often portrayed to be exacting, but is not.