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

    /**
     * Returns the start value for a task.
     *
     * @param row  the row index (zero-based).
     * @param column  the column index (zero-based).
     *
     * @return The start value.
     */
    public Number getStartValue(int row, int column) {
        Comparable rowKey = getRowKey(row);
        Comparable columnKey = getColumnKey(column);
        return getStartValue(rowKey, columnKey);