Week numbers for 2027 not following ISO8601 rule

There are two functions for calculating the week number.

WEEKNUM() calculates the literal week number where 1/1 of any given year is week 1, incrementing the week number by 1 every Sunday or Monday, depending on your settings.

There is a separate function ISOWEEKNUM() which uses ISO 8601 rules. Maybe that's the function you're looking for.

Wait, are you talking about Calendar? You posted in the Numbers forum, so I assumed you were trying to factor week numbers in a spreadsheet.

There is a setting in MacOS Settings -> Language and Region -> Calendar where you can set the calendar to ISO 8601:

and this does cause Calendar.app to show the expected Week 53/Week 1 cutover based on the ISO 8601 standard. However, iOS does not appear to have the same option, where I have only Gregorian, Japanese, or Buddhist options.

Therefore, if you're looking for a system-wide setting on iOS (as opposed to a function in a Numbers spreadsheet), it does appear that you're out of luck. There may be other calendar apps available that honor ISO 8601, or you can provide feedback to Apple asking for calendar parity with MacOS: Feedback - Calendar - Apple

> 2027 week 1 starts on Monday 4 January according the ISO8601 rule. I cannot get my phone to show the same

Show your work. Did you follow my note regarding ISOWEEKNUM() vs. WEEKNUM()?

As noted above, WEEKNUM() will start with 1 on Friday, January 1st 2027, and increment to 2 on Sunday January 3rd 2027

ISOWEEKNUM() will start week 1 on Monday January 4th 2027, just like you're expecting, with January 1-3 as being in week 53:

Are you using ISOWEEKNUM() or WEEKNUM()?