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For computational reasons, astronomical year numbering and the ISO 8601 standard designate years so that AD 1 = year 1, 1 BC = year 0, 2 BC = year −1, etc. [c] In common usage, ancient dates are expressed in the Julian calendar, but ISO 8601 uses the Gregorian calendar and astronomers may use a variety of time scales depending on the ...
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. [1] [2]
A year zero does not exist in the Anno Domini (AD) calendar year system commonly used to number years in the Gregorian calendar (nor in its predecessor, the Julian calendar); in this system, the year 1 BC is followed directly by year AD 1 (which is the year of the epoch of the era).
The year 1 BC/BCE is numbered 0, the year 2 BC is numbered −1, and in general the year n BC/BCE is numbered "−(n − 1)" [1] (a negative number equal to 1 − n). The numbers of AD/CE years are not changed and are written with either no sign or a positive sign; thus in general n AD/CE is simply n or +n. [1]
A mean calendar year is 365 + 97 / 400 ... both for years AD and BC. Thus the traditional time line is 2 BC, 1 BC, AD 1, and AD 2. ISO 8601 uses astronomical year ...
For example, a date Anno Domini (AD) 2025 becomes Anno Lucis (AL) 6025. [1] This calendar era, which would designate 4001 BC as 'year zero', was adopted in the 18th century as a simplification of the Anno Mundi era dating system used in the Hebrew calendar and borrowing from other ideas of that time regarding the year of creation.
Conversion from Gregorian calendar years to Before Present years is by starting with the 1950-01-01 epoch of the Gregorian calendar and increasing the BP year count with each year into the past from that Gregorian date. For example, 1000 BP corresponds to 950 AD, 1949 BP corresponds to 1 AD, 1950 BP corresponds to 1 BC, 2000 BP corresponds to ...
11th millennium BC · 11,000–10,001 BC 10th millennium BC · 10,000–9001 BC 9th millennium BC · 9000–8001 BC 8th millennium BC · 8000–7001 BC 7th millennium BC · 7000–6001 BC 6th millennium BC · 6000–5001 BC 5th millennium BC · 5000–4001 BC 4th millennium BC · 4000–3001 BC 40th century BC: 39th century BC: 38th century BC ...