When asked for your age, it's likely you won't slip (with the exception of a recent birthday mistake).
But for the sprawling sphere we call home, age is a much trickier matter.
Radiocarbon dating uses isotopes of the element carbon. Cosmic rays – high energy particles from beyond the solar system – bombard Earth’s upper atmosphere continually, in the process creating the unstable carbon-14. Because it’s unstable, carbon-14 will eventually decay back to carbon-12 isotopes.
Because the cosmic ray bombardment is fairly constant, there’s a near-constant level of carbon-14 to carbon-12 ratio in Earth’s atmosphere.
The Greeks consider the first Olympic Games as the beginning or 776 BC.
The Muslims count the Prophet’s departure from Mecca, or the Hegira, as their beginning at AD 662.
There are two techniques for dating in archaeological sites: relative and absolute dating.
Today’s scientists believe that answer is incorrect, but Kelvin’s calculations were in being based on logical thinking and mathematical calculation.
The Mayan calendar used 3114 BC as their reference.
More recently is the radiocarbon date of 1950 AD or before present, BP.
Meanwhile, Arthur Holmes (1890-1964) was finishing up a geology degree at the Imperial College of Science in London where he developed the technique of dating rocks using the uranium-lead method.
By applying the technique to his oldest rock, Holmes proposed that the Earth was at least 1.6 billion years old.