Day of Year · ISO Week · Julian Day

Pick a date and get the full set of calendar metadata at once: day-of-year (1–365 or 1–366), ISO 8601 week number with the correct week-year, ISO weekday (1 = Monday, 7 = Sunday), quarter, Julian Day, Unix timestamp, leap-year flag, days remaining, and how far through the year you are as a percentage.

The ISO week is the part people get wrong, and it is wrong in both directions. A year's first ISO week is the one containing its first Thursday, so early January can still belong to the previous week-year — 1 January 2023 is 2022-W52-7 — and late December can already belong to the next one: 31 December 2024 is 2025-W01-2. That is why the tool prints a week-year alongside the week number rather than assuming the calendar year. The other trap is the 53rd week: 71 years out of every 400 have one, roughly one year in six, and 2026 is among them. A report that hard-codes 52 buckets loses a week whenever that happens.

The Julian Day is shown with a trailing .5 on purpose. Julian Days begin at noon UTC, not midnight, so any date pinned to midnight lands half a day into the count — 1 January 2000 at 00:00 UTC is JD 2451544.5, and the familiar J2000.0 epoch of 2451545.0 is noon that same day. Astronomy software and historical date tables use this scale precisely because it has no months, no leap years and no calendar reform in it: it is a plain running day count back to 4713 BC in the proleptic Julian calendar.

Everything here is computed in UTC. The Unix timestamp you get is midnight UTC of the date you picked, not midnight where you are, so comparing it against a log written in local time will be off by your offset. Day-of-year numbers are also not comparable between years: day 60 is 29 February in 2024 and 1 March in 2023, and every day number after that shifts by one across a leap year boundary.