Week Number (WEEKNUM)
This is a replacement for the WEEKNUM function available with the Analysis
Toolpak.
Where the second week of the year starts on a Sunday,
=INT((A1-(DATE(YEAR(A1),1,1)-(WEEKDAY(DATE(YEAR(A1),1,1))-1)))/7)+1
Where the second week of the year starts on a Monday,
=INT((A1-(DATE(YEAR(A1),1,1)-(WEEKDAY(DATE(YEAR(A1),1,1))-2)))/7)+1