This API would have been so much less error-prone if it had used symbolic names instead - e.g. “{year}”, “{weekYear}”, maybe even something like “{year:2}” for a 2 digit year.
A lot of analytical and financial reporting has to tie activity to its ISO week, often to roll that activity up to the correct month and year. YYYY is perfect for that.
Also, I clicked on the title because I wanted to know what format Yyyy was... I feel mislead.