private void OnMarkExerciseCompletedChanged(object sender, EventArgs e)
{
ExerciseItem ex = sender as ExerciseItem;
dataStore.UpdateDueTime(ex);
dataStore.AddHistoryItem(new ExerciseHistoryItem(ex.GUIDID, ex.IsRepetitions, DateTime.Now, ex.RequiredReps, ex.RequiredTime, (int)ex.ExerciseTimeUnits));
RebuildList();
}