CREATE FUNCTION UserDateadd
    (
    @datepart nvarchar(9)
    ,@number int 
    ,@date datetime
    )
RETURNS datetime
AS
BEGIN
    DECLARE @Return datetime

    SELECT @Return = CASE @datepart
        WHEN 'year' THEN DATEADD(year,@number,@date) 
        WHEN 'yy' THEN DATEADD(year,@number,@date) 
        WHEN 'yyyy' THEN DATEADD(year,@number,@date) 
        WHEN 'quarter' THEN DATEADD(quarter,@number,@date) 
        WHEN 'qq' THEN DATEADD(quarter,@number,@date) 
        WHEN 'month' THEN DATEADD(month,@number,@date) 
        WHEN 'mm' THEN DATEADD(month,@number,@date) 
        WHEN 'm' THEN DATEADD(month,@number,@date) 
        WHEN 'dayofyear' THEN DATEADD(dayofyear,@number,@date) 
        WHEN 'dy' THEN DATEADD(dayofyear,@number,@date) 
        WHEN 'y' THEN DATEADD(dayofyear,@number,@date) 
        WHEN 'day' THEN DATEADD(day,@number,@date) 
        WHEN 'dd' THEN DATEADD(day,@number,@date) 
        WHEN 'd' THEN DATEADD(day,@number,@date) 
        WHEN 'week' THEN DATEADD(week,@number,@date) 
        WHEN 'wk' THEN DATEADD(week,@number,@date) 
        WHEN 'ww' THEN DATEADD(week,@number,@date) 
        WHEN 'weekday' THEN DATEADD(weekday,@number,@date) 
        WHEN 'wk' THEN DATEADD(weekday,@number,@date) 
        WHEN 'w' THEN DATEADD(weekday,@number,@date) 
    END
    return @Return
END