public Number getStartValue(Comparable rowKey, Comparable columnKey,
                                int subinterval) {
        Number result = null;
        int row = getRowIndex(rowKey);
        TaskSeries series = (TaskSeries) this.data.get(row);
        Task task = series.get(columnKey.toString());
        if (task != null) {
            Task sub = task.getSubtask(subinterval);
            if (sub != null) {
                TimePeriod duration = sub.getDuration();
                result = new Long(duration.getStart().getTime());
            }
        }
        return result;