C# 获得固定年月日

 /// <summary>
        /// 获得固定年月日,时和分不固定 : 2019-01-01 00:00:00
        /// </summary>
        /// <returns></returns>
        public DateTime GetFixedDateTime(string Hour = "", string Minute = "")
        {
            string time1 = DateTime.Now.ToShortTimeString(); //16:50
            //yyyy-MM-dd hh:mm:ss //2019-01-01 16:34:00
            string Ymd = "2019-01-01 ";
            if (Hour=="")
            {
                Hour = DateTime.Now.Hour.ToString();
                Minute = DateTime.Now.Minute.ToString();
            }
            return Convert.ToDateTime(Ymd + Hour + ":" + Minute + ":" + );
        }