There is only one way to calculate week numbers within a year

[vicnett]

In the ISO week date standard (ISO-8601), December 30 and 31 2019 are part of the first week of 2020.

In the US, since weeks start on Sunday, December 29 is also part of the first week of 2020.

Some systems count from 1-53, others count from 0-53.

I won't pretend to fully understand this. I just know it's another potential source of confusion.

[grinser]

I would appreciate if this one was added to the website.