Numbering Years – Calendars

Every date you use—birthdays, holidays, history—is determined by how people count years. Use your birth year as an example. We will call the year you were born Y. The next year is 2Y. Everything before you were born is BY, or before your year.

Counting Years

Ancient calendars were generally based on the beginning of a ruler’s reign; for example, the third year of Hammurabi’s rule. Most people today use the Western calendar (also known as the Gregorian calendar) to keep track of what today is, what happened yesterday, and important dates yet to come. About AD 525, a Christian monk named Dionysius Exiguus designated the year of Christ’s birth as 1. The Western calendar tells us we live in 2026, which is sometimes written AD 2026. AD refers to the Latin phrase “anno Domini,” meaning “the year of the Lord.”

BE and AD

The years before the birth of Christ are numbered backward from his birth. The year before AD 1 was 1 BC, or one year “before Christ.” When referring to dates before the birth of Christ, the higher the number, the earlier the year.

No Year Zero

We mark years from a set point known as 1. In the example we used earlier, year one began on the day you were born. Year two began on your first birthday. When identifying your age, we count completed years. If you are twelve, you are in your thirteenth year.

CE and BCE

Modern historians use CE in place of AD and BCE in place of BC. CE stands for “Common Era,” and BCE stands for “Before the Common Era.” Occasionally, you might see the C interpreted as “Christian Era” or “Current Era,” though the numbering system is the same.

The Western calendar is the standard calendar used in every country today. Other calendars serve cultural and religious purposes, but the seven-day week and twelve-month year are used around the world for everyday life—school, work, and government.

The Islamic Calendar

Muslims use the Islamic calendar for their celebrations and holy periods. The Islamic calendar is based on the cycle of the moon, so it is ten or eleven days shorter than the Western calendar. Year one begins when the Prophet Muhammad and his followers left their homes in Mecca to journey to Medina. On a Western calendar, this is AD 622. AH is an abbreviation for a phrase that means “in the year of the journey.” AH 1448 will begin on June 16, 2026.

The Hebrew Calendar

The Hebrew calendar guides Jewish religious services and holidays. The Hebrew calendar is based on the cycle of the moon and adjusted to align with the solar year. Year one begins with the traditional date of creation. Years are marked AM, an abbreviation for the Latin phrase Anno Mundi, or “the year of the world.” The Hebrew year 5787 will begin with Rosh Hashanah, the Jewish New Year, at sunset on September 11, 2026.

The Chinese Calendar

The Chinese calendar is not used to keep track of the day or the year. It is used mainly for festivals and traditional holidays. The Chinese New Year ushered in the Year of the Horse on February 17, 2026. There are celebrations to welcome the traditional new year in Chinese communities around the world, especially in cities like San Francisco and London, known for their historic and active Chinatowns.

Dionysius Exiguus

Dionysius Exiguus lived in the sixth century of the Common Era. His name can be translated as “Dennis the Humble.” Though a writer described Dionysius as “the most learned” scholar in the city of Rome, he lived a simple life as a Christian monk. A monk is a religious person who resides apart from the rest of society in a life of prayer or study.

About 525 CE, Pope John I asked Dionysius to compile a table of future Easter dates. Easter is an important Christian holiday that falls on different dates in spring, depending on the moon’s cycle. At that time, many Romans numbered years from the reign of Emperor Diocletian. Emperor Diocletian persecuted Christians, so Dionysius numbered his table from the birth of Jesus Christ instead.

Dionysius calculated that Christ was born 524 years before his time, so the monk identified the year he wrote the table as anno Domini 525, which translates to the year of the Lord 525. Later scholars studied the Christian Bible and concluded from historical records and astronomical clues that Jesus was likely born between four and six years earlier than Dionysius’ calculation.

Grouping Years

On the Western calendar, there are ten years in a decade. Decem is the Latin word for ten. The Latin term centum, meaning 100, is the basis for the word century, a period of 100 years. A millennium lasts one thousand years. The word comes from the Latin mille, meaning one thousand. This is considered the twenty-first century because counting began with year one, not year zero. At the stroke of midnight on January 1, 2000, many people celebrated the new millennium, but they were a year early, as the millennium did not actually begin until January 1, 2001.

Eras and Epochs

An era is any significant period of time. You may be in the high school era of your life, but for a geologist, an era may last millions of years. Photographs often show clothing styles that reflect the era in which they were taken.

An epoch is a specific moment that marks the beginning or end of an era. The baseball season’s epochs are Opening Day and the World Series. Your thirteenth birthday is the epoch that begins your teenage years.