Common Era or Current Era (CE) is one of the notation systems for the world's most widely used calendar era – an alternative to the Dionysian AD and BC system. The era preceding CE is known as before the Common Era or before the Current Era (BCE), while the Dionysian era distinguishes eras as AD (anno Domini, " year of Lord") and BC ("before Christ"). Since the two notation systems are numerically equivalent, "2018 CE" corresponds to "AD 2018" and "400 BCE" corresponds to "400 BC".[a] Both notations refer to the Gregorian calendar (and its predecessor, the Julian calendar). The year-numbering system utilized by the Gregorian calendar is used throughout the world today, and is an international standard for civil calendars.
The expression has been traced back to 1615, when it first appeared in a book by Johannes Kepler as the Latin usage vulgaris aerae, and to 1635 in English as "Vulgar Era".[b] The term "Common Era" can be found in English as early as 1708, and became more widely used in the mid-19th century by Jewish academics. In the later 20th century, the use of CE and BCE was popularized in academic and scientific publications, and more generally by authors and publishers wishing to emphasize secularism or sensitivity to non-Christians, by not explicitly referencing Jesus as "Christ" and Dominus ("Lord") through use of the abbreviation[c] "AD".
The year numbering system used with Common Era notation was devised by the Christian monk Dionysius Exiguus in the year 525 to replace the Era of Martyrs system, because he did not wish to continue the memory of a tyrant who persecuted Christians. He attempted to number years from an initial reference date ("epoch"), an event he referred to as the Incarnation of Jesus. Dionysius labeled the column of the table in which he introduced the new era as "Anni Domini Nostri Jesu Christi".
Numbering years in this manner became more widespread in Europe with its usage by Bede in England in 731. Bede also introduced the practice of dating years before what he supposed was the year of birth of Jesus, and the practice of not using a year zero.[d] In 1422, Portugal became the last Western European country to switch to the system begun by Dionysius.
The term "Common Era" is traced back in English to its appearance as "Vulgar Era"[e] to distinguish dates on the Ecclesiastic calendar from those of the regnal year, the year of reign of a sovereign, typically used in national law.
The first use of the Latin term vulgaris aerae[f] discovered so far was in a 1615 book by Johannes Kepler. Kepler uses it again in a 1616 table of ephemerides, and again in 1617. A 1635 English edition of that book has the title page in English – so far, the earliest-found usage of Vulgar Era in English. A 1701 book edited by John LeClerc includes "Before Christ according to the Vulgar Æra, 6". A 1716 book in English by Dean Humphrey Prideaux says, "before the beginning of the vulgar æra, by which we now compute the years from his incarnation." A 1796 book uses the term "vulgar era of the nativity".
The first so-far-discovered usage of "Christian Era" is as the Latin phrase aerae christianae on the title page of a 1584 theology book. In 1649, the Latin phrase æræ Christianæ appeared in the title of an English almanac. A 1652 ephemeris is the first instance so-far-found for English usage of "Christian Era".