Week number confusion

Just encountered this problem when I was talking with my colleague.  He thought this week was week 43 but it actually was 42 on my calendar.  Turned out, there are two ways to look at week number. First one is to simply take January first as a start of week one.  In case of 2021, January 1 is Friday.   depending of what you think as a first day of week, this year’s week 1 is only 2 days long or 3 days long, then Week 2 will start on either January 3rd(Sunday) or January 4th (Monday) Excel function =WeekNum()  uses this approach. Then there is a week number definition by …