For at least as long as people have had written records, people have used celestial effects to mark off time. This continued until half a century ago, when we developed clocks more accurate than most celestial effects and measurements. Thus, the day and the month and the year. There are two issues with these units of time: what point in the cycle to start at, and what zero value to count from. For the day, sunrise, noon, sunset, and midnight. For the month, new moon. For the year, the solstices and the equinoxes. Curiously, our calendar starts at neither a solstice nor an equinox.
What zero point should one count from? The first system was the beginning of the reign of some local leader, the year-reign system. Under this system, this year is Barack Obama 4, Queen Elizabeth II 60, David Cameron 2, Stephen Harper 7, Julia Gillard 3, John Key 5, Michael Higgins 2, Nicolas Sarkozy 6, Angela Merkel 8, Dmitry Medvedev 5, Benjamin Netanyahu (II) 4, Mustafa Abduljalil 2, King Abdullah of Saudi Arabia 8, Sheikh Khalifa bin Zayed Al Nahyan 9, Mahmoud Ahmadinejad 8, Manmohan Singh 9, Hu Jintao 11, Emperor Akihito 24, Yoshihiko Noda 2, Felipe Calderon 7, Dilma Rousseff 2, Cristina Kirchner 6, …
Our calendar’s zero point uses an estimate of when Jesus Christ was born, an estimate made by a certain Dionysius Exiguus (“Dennis the Short”) some time around 525 CE. He never told us how he arrived at his estimate, and it is most likely mistaken. If there was a historical Jesus Christ, he was likely born a few years earlier than 1 CE, around 6 to 4 BCE. BC = “Before Christ”, AD = “Anno Domini” (“In the Year of the Lord”), BCE = “Before the Christian/Common Era”, CE = “Christian/Common Era”.
But there are some other zero points we could use. Here is this year in them:
- The French Revolutionary Calendar: 220
- The Jewish calendar, from a date calculated from the Bible of the creation of the Universe: 5772
- Anno Urbi Conditae (“In the Year of the Founding of the City”), from when the legendary hero Romulus supposedly founded Rome: 2765
- The 4rd year of the 697th Olympiad, from the first of the ancient Olympics: 2788
- From when the legendary Chinese Yellow Emperor supposedly established the Chinese calendar: 4649
- Holocene era, with a zero point near the beginning of the Holocene geological epoch and the beginning of Middle Eastern agriculture: 12012
Since some of them do not start on January 1, I use the year of January 1 in that calendar.
Astronomers prefer to use a days-only calendar, because it makes calculations easier. They have used Julian Dates since the late 19th cy., where the Julian Date’s zero point January 1, 4713 BCE in the Julian calendar. The Julian Day of midnight 1 Jan 2012 is 2455927.5. Satellite trackers often subtract 2400000.5, giving a Modified Julian Day: 55927. Various computer operating systems and software represent dates internally by this strategy, using various zero points. For instance, the Unix time() function returns the number of seconds since midnight January 1, 1970: 1325376000.