Hi @Enrico Kreisig ,
I used the following formula. Week starts on Monday. Depending on your regional settings you have to replace it.
=IF(INT((StartDate-DATE(YEAR(StartDate);1;1)+(TEXT(WEEKDAY(DATE(YEAR(StartDate);1;1)-1);"d")))/7)=0;52;INT((StartDate-DATE(YEAR(StartDate);1;1)+(TEXT(WEEKDAY(DATE(YEAR(StartDate);1;1)-1);"d")))/7))
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