Week number
Under ISO 8601 a week runs Monday to Sunday and week 1 is the one containing the first Thursday of the year. So if 1 January is a Friday, that week has only three days in the new year and belongs to the previous year as week 53. It is why the week-numbering year is shown separately. It does not always match the calendar year.
ISO 8601 weeks run Monday to Sunday, and week 1 is the week containing the first Thursday of the year, so 1 January 2021 fell in week 53 of 2020. The same panel gives the day of the year, turns a week number back into its Monday, and turns day 200 into a date.
How to find a week number
Two competing conventions cause most week-number confusion. ISO 8601, used across Europe and in most software, starts weeks on Monday and defines week 1 as the one holding the first Thursday. The North American convention starts weeks on Sunday and treats the week containing 1 January as week 1. They disagree by a week for a good part of the year, so a week number means nothing without the convention it was produced under. In a spreadsheet the two live side by side: WEEKNUM defaults to the American reading and ISOWEEKNUM does not, which is why two columns of the same file can be a week apart.
The reverse lookup exposes the same rule from the other end. Because week 1 must contain the first Thursday, it usually begins in the previous December, and a year starting on a Thursday, or a leap year starting on a Wednesday, gets a 53rd week to absorb the overflow. That is also why the week-numbering year is shown separately: in early January it is often the year before, and grouping weekly reports by the calendar year instead is the standard way to lose a week of data.
Day-of-year numbers are a separate system that people meet under a borrowed name. The ISO ordinal form is written 2026-235, and it turns up in aviation records, manufacturing lot codes, satellite telemetry and the %j output of Unix date, because it sorts and subtracts without any calendar arithmetic at all. It is often called a Julian date, which it is not: the astronomical Julian Day Number counts continuously from 4713 BC and is a seven-digit figure, not one between 1 and 366. The leap year is where ordinal numbers bite. Every day number above 59 shifts by one, so day 100 is 10 April in a common year and 9 April in a leap one, and a code read against the wrong year is out by a day for ten months of it.
What people use it for
- Reporting against a weekly schedule
- Reading a European production calendar
- Planning sprints by week number
- Finding which dates a production week covers
- Reading a manufacturing lot code
- Checking how far through the year you are
- Turning an ordinal date from a telemetry or banking record back into a date
Questions
Leave the date at today and read the ISO week.
Because ISO week 1 must contain the first Thursday. A year starting late in the week belongs to the previous year final week.
Fifty-two, or fifty-three in years that start on a Thursday or are leap years starting on a Wednesday. That is a count of week numbers, not of elapsed weeks: dividing 365 days by seven gives 52.14, which is the figure the time-between-dates tool reports.
Monday under ISO 8601. The North American convention uses Sunday and numbers differently.
WEEKNUM defaults to the US convention: Sunday start, week 1 containing 1 January. Use ISOWEEKNUM to match this.
The year the week belongs to, which can differ from the calendar year of its days. Group weekly reporting by it.
Switch to the week-to-dates tab: it gives the Monday it starts on and the Sunday it ends on.
Because it is the week containing the first Thursday of January, which usually begins in the previous December.
Switch to the day-of-the-year tab. The number and the days remaining are both shown, and they always add to 365 or 366.
No, though the names overlap. This is an ordinal date. The astronomical Julian Day counts from 4713 BC and is about 2.46 million today.
10 April in a common year, 9 April in a leap year.
It is 1 March in a common year and 29 February in a leap year. Every number above 59 shifts by one.
Only in a leap year, where it is 31 December. Ask for it in a common year and the panel says so rather than quietly rolling into January.
2026-200: the year, a hyphen and the day number.
Aviation and banking records, manufacturing lot codes, satellite telemetry, and the %j output of Unix date.