Does Day of Year upload my file or text to Tool-You servers?

No. Day of Year runs in this browser tab. Files stay on the device. Limit note: Local YMD. Remaining is year length minus ordinal day.

Should I use ISO Week Number instead of Day of Year?

Use Day of Year when you need Ordinal day 1–366 plus remaining days in the year. Use ISO Week Number when you need ISO-8601 week and ISO year. They share a category (Date & time) but not the same job.

Day of Year

Ordinal day 1–366 plus remaining days in the year.

Counts days from 1 January of that year and remaining days until 31 December. Unlike ISO Week this is not a week number. Unlike Leap Year you pass a full date, not only a year. 1 January is 1. Stays in this tab.

What Day of Year does

Day of Year sits in the Date & time group on purpose. It is not ISO Week Number (ISO-8601 week and ISO year.) and not Leap Year Check (Is leap year? Plus next/previous leap years.). Choose this page when you need Ordinal day 1–366 plus remaining days in the year. If you stretch Day of Year into a neighbor’s job, the result will look wrong even though the site chrome looks the same.

How to use Day of Year

Confirm you need Day of Year, not ISO Week Number (ISO-8601 week and ISO year.). Address: /datetime/day-of-year.
Enter the numbers or dates Day of Year uses. Ordinal day 1–366 plus remaining days in the year.
Read the result as Day of Year defines it — not as Leap Year Check (Is leap year? Plus next/previous leap years.). Counts days from 1 January of that year and remaining days until 31 December. Unlike ISO Week this is not a week number. Unlike Leap Year you pass a full date, not only a year. 1 January is 1. Stays in this tab.
Day of Year is an estimate in the browser, not professional advice. Local YMD. Remaining is year length minus ordinal day.
Need a different formula? Try Leap Year Check (Is leap year? Plus next/previous leap years.) instead of stretching Day of Year.

When not to use Day of Year

Skip Day of Year when a neighbor tool matches better — start with ISO Week Number or Leap Year Check — or when the description already warns the job will fail. In the tool’s own words: Counts days from 1 January of that year and remaining days until 31 December. Unlike ISO Week this is not a week number. Unlike Leap Year you pass a full date, not only a year. 1 January is 1. Stays in this tab.

Questions about Day of Year

What does Day of Year do that other Date & time pages on this site do not?: Counts days from 1 January of that year and remaining days until 31 December. Unlike ISO Week this is not a week number. Unlike Leap Year you pass a full date, not only a year. 1 January is 1. Stays in this tab. Short version: Ordinal day 1–366 plus remaining days in the year.
Does Day of Year upload my file or text to Tool-You servers?: No. Day of Year runs in this browser tab. Files stay on the device. Limit note: Local YMD. Remaining is year length minus ordinal day.
Should I use ISO Week Number instead of Day of Year?: Use Day of Year when you need Ordinal day 1–366 plus remaining days in the year. Use ISO Week Number when you need ISO-8601 week and ISO year. They share a category (Date & time) but not the same job.
What are the limits of Day of Year?: Local YMD. Remaining is year length minus ordinal day. Also: Counts days from 1 January of that year and remaining days until 31 December. Unlike ISO Week this is not a week number. Unlike Leap Year you pass a full date, not only a year. 1 January is 1. Stays in this tab.