The sripts creates a scalar function which accepts start date and end date as the paramteres and returns diffrerence between them in weeks. It takes into account @@DATEFIRST setting and assumes start date is earlier then end date.
Difference between two dates in weeks
CREATE FUNCTION [dbo].[WeekDiff] ( @StartDate DateTime, @EndDate DateTime ) RETURNS int AS BEGIN DECLARE @StartDayOfStartWeek datetime; DECLARE @StartDayOfEndWeek datetime; SELECT @StartDayOfStartWeek = DATEADD(d, -DATEPART(dw, @StartDate) + 1, @StartDate); SELECT @StartDayOfEndWeek = DATEADD(d, -DATEPART(dw, @EndDate) + 1, @EndDate); RETURN DATEDIFF(d, @StartDayOfStartWeek, @StartDayOfEndWeek)/7; END