In this type of dating, the amount of radioactive carbon in an object is measured. As an object ages, the amount of radioactive carbon decreases at a known rate. Therefore, the amount of carbon 14 remaining can be used to determine the age of an object. This age is subtracted from the present date yielding an absolute date for the object and those closely associated with it. However, radiocarbon dates are only exact within a range of years.
Historic documents can also provide dates.
Documents from Mesopotamia and Egypt contain a list of kings, the lengths of their reigns, and various other datable information. The date of the reign of certain kings has been determined by dating artifacts associated with documents from their reign. If documents from the reign of a king date to 2300 BC, then the other kings on the list can be dated roughly as well by adding and subtracting the lengths of their reigns. In addition, kings in Mesopotamia and pharaohs in Egypt often named years after a celestial event. Documents can also provide dates based on the study of the characteristics of the writing they contain.