Calendars and time
Year Zero
Direct answer
Year Zero is the integer year placed between -1 and +1 in astronomical numbering; traditional BC-AD and BCE-CE chronology has no year zero.
Year Zero exists in astronomical numbering but not in traditional historical era notation.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- Astronomical Year Numbering, Old Style Date, Calendar, Julian Date
Year Zero exists in astronomical numbering but not in traditional historical era notation.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Year Zero
The zero-year convention became useful after zero and algebraic negative numbers entered astronomical calculation.
Why Year Zero matters
Missing this distinction creates one-year errors in intervals that cross the era boundary.
Year Zero in practice: an example
Astronomical year 0 corresponds to 1 BCE, not to an additional civil year between 1 BCE and 1 CE.
Where readings of Year Zero go wrong
It is a mistake to assume a software date library and a historian use the same rule for year zero.
Sources and editorial method
Research references used for this entry include:
- IAU SOFA: Time Scale and Calendar Tools - IAU-standard algorithms and guidance for civil calendars, Julian dates, and seven recognized astronomical time scales.
- U.S. Naval Observatory: Introduction to Calendars - Official overview of calendar systems, leap rules, eras, and historical calendar reform.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
Check the numbering convention
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More in Calendars & Timekeeping
- Before Common Era
- Common Year
- Earth Rotation Angle
- Greenwich Sidereal Time
- Mean Solar Day
- Regnal Year
- Terrestrial Time
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Common questions
What is year zero?
Year Zero is the integer year placed between -1 and +1 in astronomical numbering; traditional BC-AD and BCE-CE chronology has no year zero.
What do people get wrong about year zero?
It is a mistake to assume a software date library and a historian use the same rule for year zero.
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