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Warning: Results from using WeekNumber(Function) and DateToWeekNumber(Function) can be wrong!
In general, WeekNumber(Function) is an ISO 8601 week number calculation but yields wrong results around the year's end / beginning! The 'examples' in Help will make you aware of the fact somehow but will not explicitely tell you that something is wrong here and how it could be fixed. See our web site for a short description of calendar week calculation and a suggested fix of the problem. See my annotation to the function. Same applies to the DateToWeekNumber(Function) which is a duplicate of WeekNumber(Function).