//Metadata: 
//Programming language: Power Query M 
//Natural language: English 
//Output: date function 

//Explanation: 
//CalendarWeek is set to American by default. ISO8601Week can only be generated 
//Generates date table used in large parts of the world, except US
//see also: https://en.wikipedia.org/wiki/ISO_week_date

//How to use: 
//This function expects a valid date as argument and can be invoked as a function



= (DateColumn as date) =>
  let
    CurrentThursday = Date.AddDays(DateColumn, 3 - Date.DayOfWeek(DateColumn, Day.Monday)), 
    YearCurrentThursday = Date.Year(CurrentThursday), 
    FirstThursdayOfYear = Date.AddDays(
      #date(YearCurrentThursday, 1, 7), 
      - Date.DayOfWeek(#date(YearCurrentThursday, 1, 1), Day.Friday)
    ), 
    ISO8601Week = Duration.Days(CurrentThursday - FirstThursdayOfYear) / 7 + 1
  in
    ISO8601Week