ISO 8601 covers that. Weeks start on Mondays. The first week of the year has January 4th in it, which means that it sometimes starts on a Monday in the previous Gregorian year. This is why strftime has separate format specifiers for ISO year and ISO week year, %Y and %G: https://www.man7.org/linux/man-pages/man3/strftime.3.html#NO...
My company (left unnamed to protect the guilty) starts our week on Thursday. Or sometimes Wednesday. But definitely not on Tuesday! Except a couple of places where it's Tuesday.
It's juuuust close enough to ISO8601 where with a bit of forethought everything could have been easy.
