Find Date By Day of Year

It’s the sequential position of a calendar date in its year, counting from 1 (January 1) to 365 in a common year or 366 in a leap year. ISO 8601 calls it “ordinal date” and writes it as YYYY-DDD (e.g. 2024-256 for September 12, 2024).

Because the day-of-year shifts after Feb 28 in a leap year. Day 60 is Feb 29 in 2024 but March 1 in 2023. Day 256 is September 12 in 2024 but September 13 in 2023 – the same ordinal points to a different calendar date depending on the year’s length. The tool always asks for the year so the leap/common classification is unambiguous.

For converting calendar dates to ordinal dates – e.g. when feeding data into systems that expect day-of-year (some scientific datasets, satellite trajectories, agricultural records, batch numbering schemes). Pick “Calendar date → Day of year” and enter Y/M/D.

The reverse mode rejects it with a specific error: “February 2023 does not have a day 29.” It uses the JS Date constructor to validate – if the constructor’s output doesn’t match the input year/month/day (e.g. Feb 29, 2023 becomes Mar 1, 2023), the input is flagged as invalid.

Constructing new Date(year, 0, dayOfYear, 12, 0, 0) at noon avoids edge cases where DST shifts a midnight Date object into the previous or next day in some timezones. Noon is safely inside the target day everywhere.

JavaScript’s Date handles a vastly wider range, but proleptic Gregorian behaviour before 1582 (when the calendar was adopted) is ahistorical. 1900-2100 covers all practical use cases. The error surfaces outside the range so you can’t get silently wrong results.

The format YYYY-DDD where DDD is the zero-padded day of year. So January 1, 2024 is 2024-001; December 31, 2024 is 2024-366 (leap year). It’s a compact, unambiguous date encoding used in scientific computing, aviation logs, and POSIX shell timestamps.

No. The page is three static files (HTML, CSS, JS) and runs entirely in your browser using plain JavaScript Date arithmetic. No fetch, no XHR, no analytics, no cookies. You can disconnect from the internet after the page loads and the tool still works.