I was stupefied to see this screen in Outlook 2013:

Wrong week number

As per ISO 8601:

The ISO 8601 definition for week 01 is the week with the Gregorian year's first Thursday in it. The following definitions based on properties of this week are mutually equivalent, since the ISO week starts with Monday:

Does Microsoft have such a basic thing wrong? Can it be fixed?

  • yet another reason to stay away from week numbers. Everyone in the entire world knows when March 5 is. But week 16... Everybody has to check and now you guys are telling me there are different standards? Vor herre til hest.
    – Christian
    Mar 5, 2021 at 9:22

3 Answers 3


Yes, you can set week numbering in Outlook:

  • go to file - options
  • in the left column select "Calendar"
  • in the "work time" group there is a field "first week of year", here select "First 4-day week" (this is actually a synonym of the ISO definition)

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Outlook uses whatever you set in its calendar options. The default setting is sometimes wrong and does not necessarily correlate with your Windows region settings.

This screenshot is from Outlook 2016 and in German, but the structure should also apply to Outlook 2013

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Mac users may have noticed that their version of Outlook doesn't have the "first week of year" setting available. Instead, Outlook uses the settings configured in macOS.

Depending on your locale / region you might have the non-ISO behaviour by default. To change it, go to System Preferences –> Language & Region –> Calendar and change it from Gregorian to ISO 8601.

This setting applies to other apps as well, such as Apple's Calendar.app, as described in this answer on the Apple StackExchange.

Screenshot of macOS Language & Region settings, changing Calendar setting to ISO 8601

  • 1
    And interestingly enough, I had to do this as well as change the first day of the week to "Sunday" (it was Monday") in order for this to work.
    – benmarks
    Oct 20, 2021 at 20:14

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