UFN_DAYSINMONTH

    create function dbo.UFN_DAYSINMONTH(@month tinyint, @Year smallint)
    returns tinyint with execute as caller
    as
    begin
        return case @month 
            when  2 then 28+case when @Year%4=0 and (@Year%400=0 or @Year%100!=0) then 1 else 0 end
            when  4 then 30
            when  6 then 30
            when  9 then 30
            when 11 then 30
            else 31
        end
    end