C# 将短时间格式变长正常时间格式

DateTime dateTimeStart = TimeZone.CurrentTimeZone.ToLocalTime(new DateTime(1970, 1, 1));
                long lTime = long.Parse(time_end + "0000000");
                TimeSpan toNow = new TimeSpan(lTime);
                DateTime notifyTime = dateTimeStart.Add(toNow);