uptime.cs

TimeSpan ts = TimeSpan.FromMilliseconds(Environment.TickCount);
Console.WriteLine("{0}d {1}h {2}m {3}s", ts.Days, ts.Hours, ts.Minutes, ts.Seconds);
//---------------------------

using System.Diagnostics;
PerformanceCounter upTime = new PerformanceCounter("System", "System Up Time");
upTime.NextValue();
TimeSpan ts = TimeSpan.FromSeconds(upTime.NextValue());
Console.WriteLine("{0}d {1}h {2}m {3}s", ts.Days, ts.Hours, ts.Minutes, ts.Seconds);

//-------------------------

public TimeSpan UpTime {
   get
   {
      using (var uptime = new PerformanceCounter("System", "System Up Time"))
      {
         uptime.NextValue();   // Must call extra time before reading its value
         return TimeSpan.FromSeconds(uptime.NextValue());
      }
    }
}

//---------------------------