Normally the time range selection gets cleared if you call Update() on the server side.
If you don't do that for some reason you need to switch to JavaScript event handling and clear it manually:
TimeRangeSelectedHandling = TimeRangeSelectedHandlingType.Callback,
TimeRangeSelectedJavaScript = "dp.clearSelection(); dp.timeRangeSelectedCallBack(start, end, resource);"
Where "dp" is the Scheduler object on the client side (ID).
It clears the selection:
dp.clearSelection();
And than it invokes the server-side event handler:
dp.timeRangeSelectedCallBack(start, end, resource);"