UFN_VALIDMONTHDAY

CREATE function dbo.UFN_VALIDMONTHDAY(@monthday char(4))
    returns bit with execute as caller
    as
    begin

        if @monthday is null
            return 1

        --4 chars are required

        if len(@monthday) <> 4
            return 0

        if isnumeric(@monthday) = 0 
            return 0

        declare @Month tinyint
        set @Month = cast(left(@monthday, 2) as tinyint)
        if (@Month < 1)
            return 0
        if @Month > 12
            return 0

        declare @Day tinyint
        set @Day = cast(right(@monthday, 2) as tinyint)
        if @Day < 1 
            return 0
        if @Day > dbo.UFN_DAYSINMONTH(@Month, 2012) -- Specifying a leap year

            return 0

        return 1

    end