A.D. or AD Calendar Designation

How Christian Church History Underlies Modern Calendars

14th Century Clockworks, Salisbury Cathedral
The oldest surviving mechanical clockworks, made 1386, Salisbury Cathedral. Ben Sutherland / Flickr / CC BY 2.0

AD (or A.D.) is an abbreviation for the Latin expression "Anno Domini", which translates to "the Year of Our Lord", and equivalent to C.E. (the Common Era). Anno Domini refers to the years which followed the supposed birth year of the philosopher and founder of Christianity, Jesus Christ. For the purposes of proper grammar, the format is properly with the A.D. before the number of the year, so A.D. 2018 means "The Year of Our Lord 2018", although it is sometimes placed before the year as well, paralleling the use of B.C.

The choice of starting a calendar with the birth year of Christ was first suggested by a few Christian bishops including Clemens of Alexandria in C.E. 190 and Bishop Eusebius at Antioch, C.E. 314–325. These men labored to discover what year Christ would have been born by using available chronologies, astronomical calculations, and astrological speculation.

Dionysius and Dating Christ

In 525 C.E., the Scythian monk Dionysius Exiguus used the earlier computations, plus additional stories from religious elders, to form a timeline for Christ's life. Dionysius is the one credited with the selection of the "AD 1" birth date that we use today—although it turns out he was off by some four years. That wasn't really his purpose, but Dionysius called the years that occurred after Christ's supposed birth "The years of our Lord Jesus Christ" or "Anno Domini".

Dionysius's real purpose was trying to pin down the day of the year on which it would be proper for Christians to celebrate Easter. (see the article by Teres for a detailed description of Dionysius efforts). Nearly a thousand years later, the struggle to figure out when to celebrate Easter led to the reformation of the original Roman calendar called the Julian Calendar into the one most of the west uses today--the Gregorian calendar.

The Gregorian Reform

The Gregorian reform was established in October of 1582 when Pope Gregory XIII published his papal bull "Inter Gravissimas". That bull noted that the existing Julian calendar in place since 46 B.C.E. had drifted 12 days off-course. The reason the Julian calendar had drifted so far is detailed in the article on B.C.: but briefly, calculating the exact number of days in a solar year was nearly impossible prior to modern technology, and Julius Caesar's astrologists got it wrong by about 11 minutes a year. Eleven minutes isn't too bad for 46 B.C.E., but it was a twelve-day lag after 1,600 years.

However, in reality, the main reasons for the Gregorian change to the Julian calendar were political and religious ones. Arguably, the highest holy day in the Christian calendar is Easter, the date of the "ascension", when the Christ was said to have been resurrected from the dead. The Christian church felt that it had to have a separate celebration day for Easter than the one originally used by the founding church fathers, at the start of the Jewish Passover. 

The Political Heart of Reform

The founders of the early Christian church were, of course, Jewish, and they celebrated Christ's ascension on the 14th day of Nisan, the date of Passover in the Hebrew calendar, albeit adding a special significance to the traditional sacrifice to the Paschal lamb. But as Christianity gained non-Jewish adherents, some of the communities agitated for separating out Easter from Passover.

In 325 C.E., the Council of Christian bishops at Nicea set the annual date of Easter to fluctuate, to fall on the first Sunday after the first full moon occurring on or next after the first day of spring (vernal equinox). That was intentionally complex because to avoid ever falling on the Jewish Sabbath, Easter's date had to be based on the human week (Sunday), the lunar cycle (full moon) and the solar cycle (vernal equinox).

The lunar cycle used by the Nicean council was the Metonic cycle, established in the 5th century B.C.E., that showed that new moons appear on the same calendar dates every 19 years. By the sixth century, the ecclesiastical calendar of the Roman church followed that Nicean rule, and indeed, it is still the way the church determines Easter each year. But that meant that the Julian calendar, which had no reference to lunar motions, had to be revised.

Reform and Resistance

To correct the Julian calendar's date slippage, Gregory's astronomers said they had to "deduct" 11 days out of the year. People were told they were to go to sleep on the day they called September 4th and when they woke up the next day, they should call it September 15th. People did object, of course, but this was only one of numerous controversies slowing acceptance of the Gregorian reform.

Competing astronomers argued over the details; almanac publishers took years to adapt—the first was in Dublin 1587. In Dublin, people debated what to do about contracts and leases (do I have to pay for the full month of September?). Many people rejected the papal bull out of hand—Henry VIII's revolutionary English reformation had taken place only fifty years earlier. See Prescott for an amusing paper on the problems this momentous change caused everyday people.

The Gregorian calendar was better at counting time than the Julian, but most of Europe held off accepting the Gregorian reforms until 1752. For better or worse, the Gregorian calendar with its embedded Christian timeline and mythology is (essentially) what is used in the western world today.

Other Common Calendar Designations

  • Islamic: A.H. or AH, meaning "Anno Hegirae" or "in the year of the Hijra"
  • Hebrew: AM or A.M., meaning "Year After Creation"
  • Western: BCE or B.C.E., meaning "Before the Common Era"
  • Western: CE or C.E., meaning the "Common Era"
  • Christian-Based Western: BC or B.C., meaning "Before Christ"
  • Scientific: AA or A.A., meaning the "Atomic Age"
  • Scientific: RCYBP, meaning "Radiocarbon Years Before the Present"
  • Scientific: BP or B.P., meaning "Before the Present"
  • Scientific: cal BP, meaning "Calibrated Years Before the Present" or "Calendar Years Before the Present"


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Hirst, K. Kris. "A.D. or AD Calendar Designation." ThoughtCo, Apr. 5, 2023, thoughtco.com/christian-church-history-underlies-calendars-169928. Hirst, K. Kris. (2023, April 5). A.D. or AD Calendar Designation. Retrieved from https://www.thoughtco.com/christian-church-history-underlies-calendars-169928 Hirst, K. Kris. "A.D. or AD Calendar Designation." ThoughtCo. https://www.thoughtco.com/christian-church-history-underlies-calendars-169928 (accessed June 6, 2023).