Day-of-Year ⇄ Date & Day-of-Week Converter
1) Day-of-year → Date & Weekday
Tip: leap years allow up to 366. The tool validates the range based on the year.
2) Date → Day-of-year & Weekday
About This Tool
Quickly translate between day-of-year (DOY) and a calendar date, and see the day of week instantly. Useful for Julian/ordinal date codes, data files, project schedules, astronomy logs, and software timestamps. Calculations are performed at midnight UTC for consistency across regions.
The converter is leap-year aware (Gregorian rule: divisible by 4, except centuries not divisible by 400), so it validates input ranges per year. It’s also privacy-first—no data leaves your browser.
Tip: use Use Today / Set Today to prefill fields with today’s UTC date.
What Is “Day-of-Year” and Why It Matters
Day-of-year (DOY)—also called an ordinal date—is the running count of days within a year: 1 for 1 January, 32 for 1 February in common years, and so on, up to 365 (or 366 in leap years). It’s a compact way to reference dates without month names and is widely used in weather records, agriculture (planting/harvest windows), finance (settlement cycles), manufacturing (lot codes), and software logs.
A Short History & Standards
Modern civil dating in most of the world follows the Gregorian calendar (introduced in 1582 and
adopted by different countries at different times). The Gregorian leap-year rule keeps the calendar aligned with
the seasons: years divisible by 4 are leap years, except centuries not divisible by 400.
For data exchange, ISO 8601 formalises several date formats—one of them is the
ordinal date form YYYY-DDD (for example, 2025-256), where DDD
is the day-of-year. This is the format you’ll often see in NOAA/NASA datasets, telemetry, and
scientific archives.
“Julian Date” vs. Day-of-Year (Common Confusion)
People sometimes say “Julian date” when they mean DOY or a compact code like YYDDD.
Strictly speaking, Julian Day Number (JDN) is an astronomical count of days from a fixed epoch
(used by astronomers worldwide and not tied to the Gregorian year). If your file names or barcodes show patterns
like 2025-035 or 25035, you’re almost certainly looking at day-of-year, not JDN.
Why DOY Is Useful
- Sorting & filtering: DOY increases monotonically within a year—great for spreadsheets and scripts.
- Seasonal analytics: Compare the same “time of year” across different years, regardless of month lengths.
- Operational shorthand: Manufacturing lots, warranty periods, and project milestones often use DOY codes.
About This Converter
This tool converts day-of-year ⇄ calendar date and provides the day of week.
It recognises leap years automatically and runs at midnight UTC to avoid daylight-saving surprises.
Results are displayed in ISO format (YYYY-MM-DD) for easy copying into reports and spreadsheets.
Everything happens locally in your browser for privacy.