Everyone uses week numbers and almost nobody agrees on them
Week numbers are ordinary in European business. A delivery is due in week 32, a project plan runs from week 4 to week 12, a factory shuts for weeks 30 and 31. They are also a reliable source of quiet errors, because there are two incompatible conventions and most people have never been told which one they are using.
The ISO standard, which this tool implements, runs weeks Monday to Sunday and defines week 1 as the week containing the first Thursday of January. The North American convention runs weeks Sunday to Saturday and calls the week containing January 1st week 1. In most years these produce different numbers, and the difference is invisible until two people compare dates.
Looking something up
- Choose a mode.
- For a date, either leave the today box ticked or untick it and enter a year, month and day.
- For a week to date range, enter a week number and a week-year.
- For a full year listing, enter a calendar year.
- Run it and read the report.
The week-year is not the calendar year
This is the concept that causes the most confusion and the reason the report always shows both.
Because the standard assigns a whole week to whichever year contains its Thursday, the days at the very start and end of a calendar year can belong to the neighbouring year’s numbering. January 1st 2023 was a Sunday, and that week’s Thursday fell in December, so the date belongs to week 52 of 2022. December 31st 2018 was a Monday, and that week’s Thursday fell in January, so it belongs to week 1 of 2019.
Neither is a quirk of this tool, both are exactly what the standard specifies. But quoting a bare week number without its week-year across a year boundary is genuinely ambiguous, which is why the report prints the combined form. Say 2026-W32 and there is nothing to misread.
Some years have 53 weeks
A year has 53 ISO weeks whenever the calendar works out such that an extra Thursday falls inside it. 2020 had 53. 2026 has 53. 2021 had 52.
This matters more than it sounds because a great deal of planning software, and a great many spreadsheets, quietly assume 52. The failure appears at the very end of a long year, when a week exists that the plan has no row for, and by then it is usually a scramble.
The full-year mode states the total before it lists anything, which makes it a two second check when you are building a plan.
The fields you are not using
All the fields for all three modes are visible at once, each labelled with the mode it belongs to. That is a limitation of the option system rather than a design preference, since it cannot show and hide fields based on another field’s value.
The practical consequence is only visual. A field belonging to a mode you are not running is ignored completely, so a leftover value in the week number field cannot affect a date lookup. Reading the labels rather than the field order is the way to navigate it.
Related date tools
For the number of days between two dates rather than which week one falls in, Days Between Dates does that arithmetic, and Age Calculator is the same operation with a birthday framing.
For a physical calendar to write on, Printable Calendar generates one as a PDF. If your week numbers are being compared across offices in different countries, Time Zone Converter covers the other half of that coordination problem. The rest are on the calculators hub.
