The 365-and-change-day calendar we use is the result of scientific sweat, an attempt to bring us to a Verifiable Truth regarding how long it takes the Earth to complete one rotation around the sun. Starting a calendar 4.54 billion years ago doesn't make much intuitive sense.But while months and days are based on the planet's gravitational forces, and thereby grounded in reality, the third aspect of our dating code is a total mess. Rather, we need to find another, closer Year One to begin things. In 2012, American electrochemical engineer Libb Thims, in the overtly atheistic manuscript Purpose?
In the Year of Man's Retrospective Knowledge, by Astronomical Calculation 5000. The calendar lasted until 1806, after which Napoleon Bonaparte brought back the Gregorian calendar.
For example, 400 BCE is the same as 400 BC, and 2011 CE is the same as 2011 AD.
There is another less frequent meaning in use for the “C” in the new BCE and CE designations, in that the “C” stands for “Current,” the implication being that there is yet another era still to come.
It's a three-part system allowing those in various locations and points of time to distinguish when an event occurred or will occur. Spoilers: The planet's been around longer than any of us—or any of our ancient relatives—can remember.
The first two parts—the month and date—have had a legion of originators, from Cro-Magnon astronomers marking phases of the moon on their eagle bones, to Mayan mystics tracking the movements of the stars from their forest canopies. Tests date the Earth to about 4.54 billion years old, but a whole lot of that time didn't really have anything of substance—to us humans, at least.