Hmmm that is my function similiar to above but IMHO easier to understand

CREATE FUNCTION ISOweek(@date datetime)

RETURNS int

AS

BEGIN

declare @dayOfWeek int

declare @1ThISOWeek1ThDay datetime

declare @4ThJan datetime

declare @1ThJan datetime

declare @31ThDec datetime

declare @numberOfISOweeks int

declare @Tmp int

declare @returnValue as int

set @4ThJan = cast(CAST(YEAR(@date) AS CHAR(4)) + '-01-04' as datetime)

set @1ThJan = cast(CAST(YEAR(@date) AS CHAR(4)) + '-01-01' as datetime)

set @31ThDec = cast(CAST(YEAR(@date) AS CHAR(4)) + '-12-31' as datetime)

--"rule of thursday"

if (DATEPART(dw, @1ThJan) = 5 or DATEPART(dw, @31ThDec) = 5 )

set @numberOfISOweeks = 53

else

set @numberOfISOweeks = 52

set @dayOfWeek = (select DATEPART(dw, @4ThJan) - 1)

if @dayOfWeek = 0

set @dayOfWeek = 7

set @1ThISOWeek1ThDay = (select DATEADD(day, -(@dayOfWeek -1) ,@4ThJan))

if @date @numberOfISOweeks

set @returnValue = 1 -- or -1 as an error

return @returnValue

END