ISO Ordinal Date Converter (YYYY-DDD)
The ISO ordinal date (ISO 8601) represents a date as `YYYY-DDD`, where `DDD` is the day of the year from 001 to 366 — for example `2026-225` is the 225th day of 2026. It answers the question "which day number is this date?" directly.
Ordinal dates are compact and useful for date arithmetic: adding one day simply increments `DDD`. This converter returns the ordinal string, the raw day-of-year number, and the fraction of the year elapsed for any input date.
ISO Ordinal Date Converter (YYYY-DDD) Quick Reference
| Format | Pattern / Description | Example 1 | Example 2 |
|---|---|---|---|
| ISO Ordinal Date | YYYY-DDD | 2026-225 | 2026-001 |
* Examples are rendered in UTC for the fixed sample instants above.
When to Use ISO Ordinal Date (YYYY-DDD)
Batch data processing
ETL jobs and archive systems label daily partitions by ordinal date (e.g. `events/2026-225/`), which sorts correctly and is trivial to compute.
Day-of-year analytics
Seasonal analysis compares day 1-365 across years; convert dates to ordinals to chart yearly seasonality accurately.
Embedded systems & logs
Firmware logs and low-footprint systems frequently timestamp with year + day-of-year to save space while staying unambiguous.
Frequently Asked Questions
What is an ordinal date?
An ordinal date is year + day-of-year (1-366), written `YYYY-DDD` in ISO 8601. `2026-225` is the 225th day of 2026, which is August 13.
How is day-of-year calculated?
Day 001 is January 1; each following calendar day increments the count. December 31 is day 365 (or 366 in a leap year).
Is ordinal date the same as Julian date?
No. An ordinal date is a calendar-based day count within a year. A Julian date (JD) is a continuous count of days since a reference epoch (4713 BC) used in astronomy — see the Julian date converter.
How do I convert ordinal date back to a calendar date?
Paste `2026-225` into the converter input — the engine parses it and returns the full calendar date with all other formats.
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