Calendars and How We Count the Years

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Why do we start the calendar year in January? How do we determine when a calendar year begins?

Eras and Epochs

Most calendars begin their year at some interesting political event—such as the start of a king’s reign, a battle victory, or even the beginning of the world. Others may count the years according to rotating cycles. As an example, the Gregorian calendar, which many of us use, is based on the Christian era.

Every year, The Old Farmer’s Almanac includes a list of 10 calendar eras in its “How to Use the Right-Hand Calendar Pages” section. Below are a few examples.

Jewish Era  (a.m.)

Began: 3761 b.c.

Epoch: creation of the world

Background: Used for the Hebrew lunisolar calendar. Calculations are largely based on the Tanakh, the Hebrew Bible, as well as on Seder Olam Rabbah, a biblical chronology written in the 2nd century. The abbreviation a.m. stands for anno mundi, which is Latin for “in the year of the world.”


Roman Era  (a.u.c.)

Began: 753 b.c.

Epoch: founding of Rome

Background: Year is approximate. The abbreviation a.u.c. stands for ab urbe condita, Latin for “from the founding of the city.”

Diocletian Era

Began: a.d. 284

Epoch: accession of Roman emperor Diocletian

Background: Because of Diocletian’s substantial reforms during a failing empire, his reign was treated as a separate era from the one based on the foundation of Rome. However, the emperor was also known for his severe persecution of Christians, which led to their naming it the “Era of Martyrs.”

Christian Era

Began: a.d. 1

Epoch: Incarnation of Christ

Background: Invented by 6th-century Scythian monk Dionysius Exiguus, who sought to create a year-numbering system based on the Incarnation of Christ rather than employ the then-traditional Diocletian era. Dionysius determined that Christ was born on December 25, 753 a.u.c. (Roman era), so he used January 1, 754 a.u.c. as the beginning of the Christian era and dated the year a.d. 1. (Many scholars put the birth date earlier, such as in 4 b.c.) The prefix a.d. stands for anno Domini, Latin for “in the year of the Lord.”


Dionysius Exiguus, Two tracts on Paschal reckoning: fols. 135v-138v

The idea of “before Christ” did not appear until at least the 8th century, when the English monk Venerable Bede expanded Dionysius’s system, designating the period before the epoch as ante uero incarnationis Dominicae tempus, Latin for “before the time of the true Incarnation of the Lord.” Centuries after Bede’s system caught on, the Latin was translated to the English “before Christ” and abbreviated as b.c.

About The Author

Heidi Stonehill

Heidi Stonehill is the executive editor for The Old Farmer’s Almanac, where she focuses much of her time on managing content development for the Almanac’s line of calendars. Read More from Heidi Stonehill

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