Chronology is the science of locating events in time. Methods of determining chronology are used in most disciplines of science, especially astronomy, geology, history, paleontology and archaeology.
Basic chronological acronyms and calendars
AM or A.M.
Anno Mundi from Latin "in the year of the world" lit. "to the creation of the world", or "Year After Creation", is a calendar era based on the biblical accounts of the creation of the world and subsequent history. Two such calendar eras of notable use are:
The Hebrew calendar based since the Middle Ages on rabbinic calculations of the year of creation from the Hebrew Masoretic Text of the Bible. In the Hebrew calendar, the day begins at sunset. The calendar's epoch, corresponding to the calculated date of the world's creation, is equivalent to sunset on the Julian proleptic calendar date 6 October 3761 BCE. Anno mundi 5785 (meaning the 5,785th year since the creation of the world) began at sunset on 3 October 2024 according to the Gregorian calendar.
The Creation Era of Constantinople or the Era of the World (Ancient Greek: Ἔτη Γενέσεως Κόσμου κατὰ Ῥωμαίους, also Ἔτος Κτίσεως Κόσμου or Ἔτος Κόσμου; lit. 'Roman year since the creation of the universe', abbreviated as ε.Κ.), was the calendar used by the Eastern Orthodox Church from c. 691 to 1728 in the Ecumenical Patriarchate. It was also the official calendar of the Byzantine Empire from 988 to 1453 and it was used in Russia until 1700. This calendar was used also in other areas of the Byzantine commonwealth such as in Serbia.
BC
The terms anno Domini (AD) and before Christ (BC) are used when designating years in the Gregorian and Julian calendars. The term anno Domini is Medieval Latin and means "in the year of the Lord" taken from the full original phrase "anno Domini nostri Jesu Christi", which translates to "in the year of our Lord Jesus Christ". The form "BC" is specific to English, and equivalent abbreviations are used in other languages: the Latin form, rarely used in English, is ante Christum natum (ACN) or ante Christum (AC).
Calendar Era
A calendar era is the period of time elapsed since one epoch of a calendar and, if it exists, before the next one. For example, the current year is numbered 2025 in the Gregorian calendar, which numbers its years in the Western Christian era (the Coptic Orthodox and Ethiopian Orthodox churches have their own Christian eras).
In antiquity, regnal years were counted from the accession of a monarch. This makes the chronology of the ancient Near East very difficult to reconstruct, based on disparate and scattered king lists, such as the Sumerian King List and the Babylonian Canon of Kings. In East Asia, reckoning by era names chosen by ruling monarchs ceased in the 20th century except for Japan, where they are still used.
CE and BCE
Common Era (CE) and Before the Common Era (BCE) are year notations for the Gregorian calendar (and its predecessor, the Julian calendar), the world's most widely used calendar era. Common Era and Before the Common Era are alternatives to the original Anno Domini (AD) and Before Christ (BC) notations used for the same calendar era. The two notation systems are numerically equivalent: "2025 CE" and "AD 2025" each describe the current year; "400 BCE" and "400 BC" are the same year.
Epoch
In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured.
The moment of epoch is usually decided by congruity, or by following conventions understood from the epoch in question. The epoch moment or date is usually defined from a specific, clear event of change, an epoch event. In a more gradual change, a deciding moment is chosen when the epoch criterion was reached.
Olympiad
An olympiad (Greek: Ὀλυμπιάς, Olympiás) is a period of four years, particularly those associated with the ancient and modern Olympic Games.
Although the ancient Olympics were established during Greece's Archaic Era, it was not until Hippias that a consistent list was established and not until Ephorus in the Hellenistic period that the first recorded Olympic contest was used as a calendar epoch. Ancient authors agreed that other Olympics had been held before the race won by Coroebus but disagreed on how many; the convention was established to place Coroebus's victory at a time equivalent to the summer of 776 BC in the Proleptic Julian calendar, and to treat it as Year 1 of Olympiad 1. Olympiad 2 began with the next games in the summer of 772 BC.
Thus, for N less than 195, Olympiad N is reckoned as having started in the year 780-(4 x N) BC and ended four years later. For N greater than or equal to 195, Olympiad N began in AD (4 x N)-779 and ended four years later. By extrapolation, the 1st year of the 701th Olympiad begins roughly around 2 August 2025.
YBP and ybp
Before Present (BP) or "years before present (YBP)" is a time scale used mainly in archaeology, geology, and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Because the "present" time changes, standard practice is to use 1 January 1950 as the commencement date (epoch) of the age scale, with 1950 being labelled as the "standard year". The abbreviation "BP" has been interpreted retrospectively as "Before Physics", which refers to the time before nuclear weapons testing artificially altered the proportion of the carbon isotopes in the atmosphere, which scientists must account for when using radiocarbon dating for dates of origin that may fall after this year.
SE
The Seleucid era or Anno Graecorum (literally "year of the Greeks" or "Greek year"), sometimes denoted "AG," was a system of numbering years in use by the Seleucid Empire and other countries among the ancient Hellenistic civilizations, and later by the Parthians. It is sometimes referred to as "the dominion of the Seleucidæ," or the Year of Alexander. The era dates from Seleucus I Nicator's reconquest of Babylon in 312/11 BC after his exile in Ptolemaic Egypt, considered by Seleucus and his court to mark the founding of the Seleucid Empire. According to Jewish tradition, it was during the sixth year of Alexander the Great's reign (lege: possibly Alexander the Great's infant son, Alexander IV of Macedon) that they began to make use of this counting.
