Day of year
N = days of the months before + day of the month
Day 1 is 1 January. From March on, a leap year adds one: day 60 is 1 March in a common year and 29 February in a leap year.
// CONVERTER
Day of year, date and week numbers.
IRIG time codes and recorder time stamps count days from 1 January, not months: day 278 is a date nobody remembers. Type a day of year to get the date, or a date to get its day of year, with the ISO week, the GPS week and the Julian Day Number. Gregorian calendar, UTC.
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RESULTS
Date
Monday, 5 October 2026
2026-10-05
Day of year
Day 278
of 365 · 87 days left
| Calendar | |
|---|---|
| ISO 8601 date | 2026-10-05 |
| ISO 8601 ordinal date | 2026-278 |
| ISO week | Week 41 of 2026 (2026-W41-1) |
| Leap year | No: 365 days |
| Time codes | |
| IRIG-B and Chapter 10 day | 278 |
| GPS week | 2,439, day 1 (Monday) |
| GPS week, 10-bit count | 391 · next rollover 21 November 2038 |
| Julian Day Number (astronomy) | 2,461,319 |
Check the highlighted field: every box needs a valid number.
HOW IT IS COMPUTED
Counting days, four ways.
Each count starts somewhere else: 1 January, the Monday of the first week, 6 January 1980, or 1 January 4713 BC.
Leap year
divisible by 4, except centuries not divisible by 400
2024 and 2000 are leap years, 2100 is not. Getting it wrong shifts every date after February by one day.
ISO week
week 1 = the week that holds the year's first Thursday
Weeks start on Monday. The first days of January can belong to the last week of the year before, and the last days of December to week 1 of the next.
GPS week
W = ⌊(date − 6 January 1980) / 7 days⌋
GPS counts weeks from 6 January 1980, Sunday being day 0. The navigation message sends the week on 10 bits, so the count rolls over every 1,024 weeks, about 19.6 years: in August 1999, in April 2019, next in November 2038. GPS time runs ahead of UTC by the leap seconds added since 1980, so the GPS day starts a few seconds before UTC midnight.
Julian Day Number
days since 1 January 4713 BC (Julian calendar), at noon
The astronomers' continuous count: 2,461,319 for 5 October 2026. A "Julian date" in a test report usually means the day of year, a different number: say which one you mean.
The day is a UTC day. A time code set to local time, as some ranges do, changes day at local midnight.
Further reading
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