Calendars and time
Ordinal Date
Direct answer
Ordinal Date: An Ordinal Date identifies a day by its calendar year and sequential day number from 001 through 365 or 366.
An Ordinal Date is a year plus its numbered day of year.
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At a glance
- Found in
- Calendars & Timekeeping
- Related
- ISO Week Date, Week Number, Calendar, Julian Date
An Ordinal Date is a year plus its numbered day of year.
A cycle or timeline schematic for comparing the named timekeeping concept.
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The story behind Ordinal Date
Day-of-year notation became common in technical systems that needed compact, sortable dates independent of month names.
Why Ordinal Date matters
Ordinal dates simplify elapsed-day calculations and avoid month-length differences in scientific and operational records.
How to read Ordinal Date
Ordinal Date speaks to observations, mission logs, manufacturing codes, and datasets organized by day of year.
Ordinal Date in practice: an example
The sixtieth day is March 1 in a common year but February 29 in a leap year.
Where readings of Ordinal Date go wrong
It is a mistake to read an ordinal day without checking whether the year is common or leap.
Sources and editorial method
Research references used for this entry include:
- ISO: ISO 8601 Date and Time Format - Standards-body overview of unambiguous international date and time representations.
- Source
- The Great Almanac Dictionary
- Category
- Calendars & Timekeeping
- Last updated
- 2026-09-26
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More in Calendars & Timekeeping
- Sidereal Year
- Unix Time
- Astronomical Day
- Callippic Cycle
- Draconic Year
- Golden Number
- International Atomic Time
- Local Mean Solar Time
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Common questions
What is an Ordinal Date?
An Ordinal Date identifies a day by its calendar year and sequential day number from 001 through 365 or 366.
What do people get wrong about an Ordinal Date?
It is a mistake to read an ordinal day without checking whether the year is common or leap.
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