calculate the number of the week based on a date in a list?

Week starts on monday. Depending on your regional settings you have to replace the ; with ,

Example:

=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))


Example:

=INT(([Start Date]-DATE(YEAR([Start Date]),1,1)+(TEXT(WEEKDAY(DATE(YEAR([Start Date]),1,1)),”d”)))/7)+1