Bonus problem: in my company, the business calendar (which is the first week of the year, quarter ends) is sometimes shifted by one week with respect to the ISO

Bonus problem: in my company, the business calendar (which is the first week of the year, quarter ends) is sometimes shifted by one week with respect to the ISO definitions for historic reasons. I have implemented a large application which does a lot of time calculations, and run onto most of the issues listed in the article, except that I didn't use hewbrew calendars :).