c# 日期格式转换

/// <summary>
        /// 日期转换
        /// </summary>
        /// <param name="date">要改变的日期</param>
        /// <returns>以“9999年99月99日“格式返回的日期</returns>
        public static string getYmd(string date)
        {
            if (string.IsNullOrEmpty(date) == true) return "";
            string value;
            if (date.Length == 17)
            {
                value = date.Substring(0, 8);
            }
            else if (date.Length == 18)
            {
                value = date.Substring(0, 9);
            }
            else if (date.Length == 19)
            {
                value = date.Substring(0, 10);
            }
            else
            {
                value = date;
            }
            // 格式为9999/99/99的时候，Convert(varchar(10),date,111)
            if (Regex.Match(value, @"^/d{4}//d{1,2}//d{1,2}").Success == true)
            {
                string temp_month = (value.Substring(5, value.LastIndexOf("/") -
                value.IndexOf("/") - 1).Length == 1) ? "0" + value.Substring(5,
                value.LastIndexOf("/") - value.IndexOf("/") - 1) :
               (value.Substring(5, value.LastIndexOf("/") - value.IndexOf("/") - 1));
                string temp_day = (value.Substring(value.LastIndexOf("/") + 1,
                value.Length - value.LastIndexOf("/") - 1).Length == 1) ? "0" +
                value.Substring(value.LastIndexOf("/") + 1, value.Length - value.LastIndexOf("/") - 1) :
                value.Substring(value.LastIndexOf("/") + 1, value.Length - value.LastIndexOf("/") - 1);
                return value.Substring(0, 4) + "年" + temp_month.Trim() + "月" + temp_day.Trim() + "日";
            }
            // 格式为9999/99/99的时候，Convert(varchar(10),date,121)
            if (Regex.Match(value, @"^/d{4}-/d{1,2}-/d{1,2}").Success == true)
            {
                string temp_month = (value.Substring(5, value.LastIndexOf("-")
                - value.IndexOf("-") - 1).Length == 1) ? "0" +
                value.Substring(5, value.LastIndexOf("-") - value.IndexOf("-") - 1)
               : (value.Substring(5, value.LastIndexOf("-") - value.IndexOf("-") - 1));
                string temp_day = (value.Substring(value.LastIndexOf("-") + 1, value.Length -
               value.LastIndexOf("-") - 1).Length == 1) ? "0" +
               value.Substring(value.LastIndexOf("-") + 1, value.Length - value.LastIndexOf("-") - 1) :
               value.Substring(value.LastIndexOf("-") + 1, value.Length - value.LastIndexOf("-") - 1);
                return value.Substring(0, 4) + "年" + temp_month.Trim() + "月" + temp_day.Trim() + "日";
            }

            return "";
        }