Ok, I know this is simple - but the snippet was in my archive so I'm going to post it. These two functions will return the first and/or last day of the month of the passed in datetime.

• Example: SELECT dbo.GetFirstDayOfMonth(GETDATE())

• Returns: 2013-01-02 00:00:00.000

• Example: SELECT dbo.GetLastDayOfMonth(GETDATE())

• Returns: 2013-02-28 23:59:59.990

CREATE FUNCTION GetFirstDayOfMonth
(
	@DateTime DateTime
)
RETURNS DateTime
AS
BEGIN
	RETURN Convert(DateTime, '1/' + Convert(VarChar, Month(@DateTime)) + '/' + Convert(VarChar, Year(@DateTime)) +  ' 0:0:0.0')
END
}}

{{code(sql)
CREATE FUNCTION GetLastDayOfMonth
(
	@DateTime DateTime
)
RETURNS DateTime
AS
BEGIN
	RETURN Convert(DateTime, Convert(VarChar, (DateAdd(MM, 1, @DateTime - Day(@DateTime) + 1) - 1), 101) + ' 23:59:59.99')
END