For example, Christian time counts the birth of Christ as the beginning, AD 1 (Anno Domini); everything that occurred before Christ is counted backwards from AD as BC (Before Christ).The Greeks consider the first Olympic Games as the beginning or 776 BC.
More recently is the radiocarbon date of 1950 AD or before present, BP.
There are two techniques for dating in archaeological sites: relative and absolute dating.
The Muslims count the Prophet’s departure from Mecca, or the Hegira, as their beginning at AD 662.
The Mayan calendar used 3114 BC as their reference.
His radiocarbon dating technique is the most important development in absolute dating in archaeology and remains the main tool for dating the past 50,000 years.
How It Works: Carbon has 3 isotopic forms: Carbon-12, Carbon-13, and Carbon-14.
The numbers refer to the atomic weight, so Carbon-12 has 6 protons and 6 neutrons, Carbon-13 has 6 protons and 7 neutrons, and Carbon-14 has 6 protons and 8 neutrons.
The extra neutrons in Carbon-14’s case make it radioactive (thus the term, radiocarbon).
Half-lives vary according to the isotope, for example, Uranium-238 has a half-life of 4500 million years where as Nitrogen-17 has a half-life of 4.173 seconds!