Apple - Standard (ISO 8601) week number in Calendar.app?

Calendar System

System Preferences in Yosemite 10.10.5 (and all later OS X releases, through El Capitan 10.11.3) lets you change the calendar system used across apps. The default for a United States install is Gregorian. To get behavior compliant with the ISO 8601 standard, change to ISO 8601.

Choose System Preferences > Language & Region > Calendar (popup menu) > ISO 8601.

Below are screen shots of Apple’s Calendar.app, before and after switching between Gregorian calendar and ISO 8601 calendar. I added the "Gregorian"/"ISO" labels.

Note the week numbers shown on the left. Weeks 52 & 1 changed to weeks 52 & 53. In a standard week, January 1st of 2016 is in Week 53. In ISO 8601 notation for weeks, that would be the week 2015-W53, and January 1st 2016 would be 2015-W53-5 with the weekday number appended.

For more info, see an entire Wikipedia article dedicated to ISO week date.

CAVEAT I do not know the ramifications of changing your Mac calendar system from Gregorian to ISO 8601. I searched support.apple.com but found no documentation.