关机时间，引用：https://www.coder.work/article/1589448

  public static DateTime GetLastSystemShutdown()
        {
            string sKey = @"System\CurrentControlSet\Control\Windows";
            Microsoft.Win32.RegistryKey key = Microsoft.Win32.Registry.LocalMachine.OpenSubKey(sKey);

            string sValueName = "ShutdownTime";
            byte[] val = (byte[])key.GetValue(sValueName);
            long valueAsLong = BitConverter.ToInt64(val, 0);
            return DateTime.FromFileTime(valueAsLong);
        }

开机时间：

 public static DateTime GetStartTime()
        {
            DateTime n = DateTime.Now;
            foreach (var o in new ManagementObjectSearcher("SELECT  LastBootUpTime  FROM Win32_OperatingSystem").Get())
            {

                var obj = (ManagementObject)o;       
                var LastBootUpTime = obj.GetPropertyValue("LastBootUpTime").ToString();
                LastBootUpTime = LastBootUpTime.Split('.').FirstOrDefault(); 
                System.IFormatProvider format = new System.Globalization.CultureInfo("zh-CN", true);
                //string strDateFormat = "yyyyMMdd";
                n = DateTime.ParseExact(LastBootUpTime, "yyyyMMddHHmmss", format, DateTimeStyles.AllowWhiteSpaces);
                break;
            }
            return n;
        }