t-sql first last day date of the month

-- =============================================
-- Author:Bernabe Diaz
-- Description:returns the first or last day date
-- of the month
-- @d // the date ; if null is the current date
-- @FL first or last date day of the month
-- for 1 is the first day otherwise the last
-- =============================================
CREATE PROCEDURE Usp_FirstLastMonthDayDate 
@d AS DATETIME=NULL,
@FL AS INT=1,
@R AS DATETIME OUT
AS
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;

IF @d IS NULL
BEGIN
SELECT @d=GETDATE()
END

IF @FL=1
BEGIN
-- Return the first day date
SELECT @R=dateadd(day,-(datepart(day,@d)-1),@d)
END 
ELSE
BEGIN
-- Return the last day date
SELECT @R=dateadd(day,-(datepart(day,dateadd(mm,1,@d))),dateadd(mm,1,@d))
END

SELECT @R
END