cxiは閏年の方法コードかどうかを簡単に判断します。

/// <summary>
        ///       ？
        /// </summary>
        /// <param name="a_year"></param>
        /// <returns></returns>
        public static bool IsLeapYear(int a_year)
        {
            String s;
            DateTime dt;
            try
            {
                s = (a_year.ToString() + "-02-29");
                dt = Convert.ToDateTime(s);
                return true;
            }
            catch
            {
                return false;
            }
        }