CREATE FUNCTION [dbo].[f_get_first_date_of_month] 
(
    @Date DATETIME
)
RETURNS DATETIME
AS
BEGIN

   return DATEADD(d, 0, DATEADD(m, DATEDIFF(m, 0, @Date), 0))
END

CREATE FUNCTION [dbo].[f_get_last_date_of_month] 
(
    @Date DATETIME
)
RETURNS DATETIME
AS
BEGIN

    RETURN DATEADD(d, -1, DATEADD(m, DATEDIFF(m, 0, @Date) + 1, 0))

END