The core issue is that the purpose of a calendar is to track astronomical events, and in particular, their relations. The three that are universally tracked are the three that are obvious to anyone:
The rotation of the Earth
The movement of the Moon around the Earth
The movement of the Earth around the Sun.
The issue is that these time periods do not ...
I finally tracked down what seems to be a copy of the letter that I think you are referring to on Find a Grave. The extract of the relevant section is here:
(Click to enlarge)
The text reads:
A testimony concerning that Faithful Servant of the Lord, & my Dear Friend and Kinsman Cuthbert Hayhurst, who was born at Easington in Bolland in the County ...
When did humans start to keep track of years with unique identifiers for each year? [...]
My questions is: when was that "later point"? When did we start "numbering" the years to keep track of them? Who did it first? [...]
Also, when did a large portion of a population become aware of this (the numbering)?
It'll probably be impossible to know. ...
When did humans start to keep track of years with unique identifiers for each year ?
There are two possible ways of approaching this question, depending on what exactly it is you are actually asking :
1. When did humans start to keep track of years with unique identifiers for each year ?
Two random examples:
The Greek philosopher Plato relates in two ...
Julian calendar was introduced in 45 BC. Leap years are those whose number is divisible by 4. However, there is a catch here. Of course Julius did not count the (negative) years BC (as we do).
(There is a joke: "Archeologists found a coin with the date inscribed: 45 BC":-)
To preserve the pattern of divisibility by 4, one needs
to count from zero. But ...