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

Is there a way to have Calendar.app display its week numbers using the standard ISO 8061 week definition?

The standard says a week starts on Monday, and the first Thursday of the year defines the first week. So at the end of 2015, the standard week of 2015-12-28 to 2016-01-03 is Week 53. In contrast, Calendar.app in Yosemite & Mountain Lion shows Week 1.

I'm wondering:

• What definition of week is Apple using? Perhaps simply the First of January is always Week 1?
• Is there any way to change this default behavior to show standard weeks?

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.

Apple use the standard calendar "Gregorian calendar".

In contrast the:

ISO week-numbering year (also called ISO year informally) has 52 or 53 full weeks. That is 364 or 371 days instead of the usual 365 or 366 days. The extra week is referred to here as a leap week, although ISO 8601 does not use this term. Weeks start with Monday. The first week of a year is the week that contains the first Thursday of the year (and, hence, always contains 4 January). ISO week year numbering therefore slightly deviates from the Gregorian for some days close to 1 January.

Anyway, if you want ISO week number use this link to install it.