Not wanting to lleave the topic closed with incorrect info.. here are the Corrections.. :

Alter FUNCTION [dbo].[FctDateTime_FirstDayOfWeek2]

(

-- Add the parameters for the function here

@Date date

)

RETURNS date

with schemabinding

AS

BEGIN

declare @firstDate date

declare @Daysdiff int = dateDiff(dd,(datefromparts('1900','01','01')),@Date)%7 --- ***Correct line. Cast is not determenistic

select @firstDate = DATEADD(dd, @Daysdiff*-1, @Date)

RETURN @firstDate

END

OR

Alter FUNCTION [dbo].[FctDateTime_FirstDayOfWeek2]

(

-- Add the parameters for the function here

@Date date

)

RETURNS date

with schemabinding

AS

BEGIN

declare @firstDate date

declare @Daysdiff int = dateDiff(dd,(CONVERT(datetime, '1900-01-01', 101)),@Date)%7 ---*** Changed line. Convert is determenistic.

select @firstDate = DATEADD(dd, @Daysdiff*-1, @Date)

RETURN @firstDate

END