Display Date Suffix in C#

private string GetDateSuffix(string day)

{

    string strDatesuffix = "th";

    if (int.Parse(day) < 11 || int.Parse(day) > 20)

    {

        day = day.ToCharArray()[day.ToCharArray().Length - 1].ToString();

        switch (day)

        {

            case "1":

                strDatesuffix = "st";

                break;

            case "2":

                strDatesuffix = "nd";

                break;

            case "3":

                strDatesuffix = "rd";

                break;

        }

    }

    return strDatesuffix ;

}