Calculate calendar week from a date field

If you want to calculate the calendar week number from a given date field you should create a new column of type "Calculated (calculation based on other columns)" and apply the following formula to the field, assuming that your date field column is called "Datum":

=IF(Datum<>"";(ROUND((Datum-DATE(YEAR(Datum+3-MOD(Datum-2;7));1;MOD(-2;7)-9))/7;0));"")