Calculate no days in a particular month

DECLARE @today_date DATETIME
SET @today_date = '2009/02/01'

SELECT DAY(DATEADD(d, -DAY(DATEADD(m,1,CAST(CAST(YEAR(@today_date) AS VARCHAR(4)) + '-'
+ (CAST(MONTH(@today_date)AS VARCHAR(6)) + '-01') AS SMALLDATETIME))),DATEADD(m,1,CAST(CAST(YEAR(@today_date) AS VARCHAR(4)) + '-'
+ cast(MONTH(@today_date) AS VARCHAR(5)) + '-01' AS SMALLDATETIME))))