My Query And User Defined Fuction Problem

I have a table which has six fields ID, dateDue, dateReceived, dueday. month, ContactFYE
my query looks like

select ID, DateDue, dateReceived, dueday, month, ContactFYE
from Report
where (dbo.Report.DateDue BETWEEN dbo.udfDisplayTime(dueday, month, ContactFYE) AND DateDue )

user defined function

CREATE FUNCTION dbo.udfDisplayTime ( @dueday int, @month int, @ContactFYE smalldatetime)
RETURNS smalldatetime AS
BEGIN
DECLARE @ReturnString AS smalldatetime
Declare @dueday1 as int
if (@month =1)
begin
set @dueday1 = @dueday -7
end
else if (@month =2)
begin
set @dueday1 = @dueday -14
end
else if (@month =3)
begin
set @dueday1 = @dueday -60
end

select @ReturnString = DATEADD ( dd, @dueday1, @ContactFYE)
Return @ReturnString

END


I got a incorrect result when using this query and user defined function.
the result that I got was out of between displaytime and datdue.

any idea for this , Thanks.