C#

private static readonly object _syncLock = new object();
private bool _cancel;
private void ScheduleProcess(DateTime scheduledDate)
{
  if (scheduledDate >= DateTime.Now)
    throw new ArgumentOutOfRangeException("The scheduled time is set for a time that has already passed.");
  TimeSpan waittime = scheduledDate - DateTime.Now;
  lock (_syncLock)
  {
    Monitor.Wait(_syncLock, waittime);
  }
}

public void Cancel()
{
  _cancel = true;
  lock(_syncLock)
  {
    Monitor.PulseAll(_syncLock);
  }
}

public void DoSomething(DateTime scheduledTime)
{
  SmartThreadPool stp = new SmartThreadPool();
  stp.QueueWorkItem(new WorkItemCallback(RunProcess), scheduledTime);
}

private object RunProcess(object scheduledTime)
{
  ScheduleProcess((DateTime)scheduledTime);
  if (!_cancel)
  {
    // Run my process
  }

  return null;
}