Counting ExecutionTime may not be safe to date changes

[abeham]

I'm not entirely sure about this and it's a rare case, but since we're using local time to measure it could be possible that we obtain one measurement before a time zone or daylight savings change and the other afterwards. The next daylight savings change is on October 30th, 2011.

We should either use ​DateTime.UtcNow or the ​Stopwatch class.

    private void timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e) {
      System.Timers.Timer timer = (System.Timers.Timer)sender;
      timer.Enabled = false;
      DateTime now = DateTime.Now;
      ExecutionTime += now - lastUpdateTime;
      lastUpdateTime = now;
      timer.Enabled = true;
    }

I think we should use the Stopwatch for this kind of task.

[abeham]

r9343: changed date difference calculation to use UTC dates to avoid wrong time spans around daylight savings changes

[mkommend]

Reviewed r9343.