I’m familiar with Gregorian vs. ISO-8601 year numbering differences (related to YYYY vs. yyyy issues), but had no idea the two calendars defined week numbering differently. Realized this when planning travel with a European colleague - his version of Outlook is set to ISO-8601 week numbering, mine is inheriting from macOS Language and Region settings which default to Gregorian calendar in the US. Gah!
I can’t find a list of other settings that will change between these calendars, so I’m unsure about making the change. On top of that, iOS 15 doesn’t support ISO-8601 as an option (at least in the US!).