Calculate number of days in month in sql server

Create simple Scalar - Valued  function (UDF) with input parameter month & Year

Create FUNCTION [dbo].[Days_in_Month]
    (
    @month int,
    @year int
   
    )
RETURNS int
AS
BEGIN
   


DECLARE @Days int

 select @Days= datepart(dd,dateadd(dd,-1,dateadd(mm,1,cast(cast(@year as varchar)+'-'+cast(@month as varchar)+'-01' as datetime))))



    RETURN @Days
END

Run following script in Query Editor:
SELECT dbo.Days_in_Month(1,2012) as NumDaysInMonth