Stratigraphic excavation is the recording and study of these different strata as they are removed from the area.
If archaeologists know how pottery styles, glazes, and techniques have changed over time they can date sites based on the ratio of different kinds of pottery.
This number is usually written as a range, with plus or minus 40 years (1 standard deviation of error) and the theoretical absolute limit of this method is 80,000 years ago, although the practical limit is close to 50,000 years ago.
Because the pool of radioactive carbon in the atmosphere (a result of bombardment of nitrogen by neutrons from cosmic radiation) has not been constant through time, calibration curves based on dendrochronology (tree ring dating) and glacial ice cores, are now used to adjust radiocarbon years to calendrical years.
They do not, however, give "absolute" dates because they merely provide a statistical probability that a given date falls within a certain range of age expressed in years.
Chronometric methods include radiocarbon, potassium-argon, fission-track, and thermoluminescence.