What is an ISO 8601 week number?
It is the week numbering of the ISO 8601 standard. Weeks run Monday to Sunday, and week 1 is the week with the year's first Thursday (the same as the week containing January 4). A year has 52 or 53 weeks; it has 53 when January 1 is a Thursday, or a Wednesday in a leap year.
Why do dates around New Year belong to another year's week?
ISO 8601 does not split weeks at the new year, so December 29–31 can be in week 1 of the next year, and January 1–3 can be in week 52 or 53 of the previous year. For example, Monday, December 30, 2024 is in week 1 of 2025, and Friday, January 1, 2021 is in week 53 of 2020. This page shows which year the week belongs to.
Why does my Sunday-start calendar show a different week number?
Many calendars, especially in the US, start weeks on Sunday and number them so that the week with January 1 is week 1. That is the “Week counted from Jan 1” on this page, split by the day you choose (Excel's WEEKNUM(date,1) for Sunday, WEEKNUM(date,2) for Monday; ISOWEEKNUM matches ISO 8601). Because week 1 is chosen differently, within the same year it equals the ISO number or is larger by up to 1 (Monday start) or up to 2 (Sunday start). For example, Sunday, January 10, 2021 is ISO week 1 but week 3 counted from Sunday. The first few days of a year can belong to the previous ISO year, and the count can reach week 53 or 54.
How is the week of the month counted?
The week containing the 1st of the month is week 1, and weeks are split by the start day you choose — the same as the row on a calendar. Names like “third Tuesday” count which Tuesday of the month it is, regardless of the week start, so they can differ. Both are shown.
How are the day of the year, days left and percent counted?
January 1 is day 1. Days left do not include the selected day; the progress includes it. Leap years have 366 days. “Today” is the date in the time zone set on this device.