Programmers’ Day is DOY 256
The 256th day of the year (13 Sept, or 12 Sept in leap years) is celebrated by developers because 256 = 28.
Nerdy holiday
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.
5 Fun Facts about Day-of-Year
NASA loves ordinal stamps
Mission logs often use YYYY-DDD (ISO ordinal date) so timestamps stay timezone-proof from Earth to orbit.
Space-proof dating
Leap-day swaps DOY 60
In common years, day 60 is 1 March; in leap years, day 60 is 29 February and 1 March slides to day 61.
Calendar quirks
Lot codes hide DOY
Manufacturing stamps like 25035 decode to the 35th day of 2025—handy for tracing batches and warranties.
Hidden timestamps
Perfectly sortable
Ordinal dates sort cleanly as text (2025-001 … 2025-366), so logs stay in order without extra parsing.
Spreadsheet friendly