Make sure that the Time-scale option is selected on the Axes tab (Chart menu, Chart Options command). If this option isn't available, either you aren't using the correct chart type or you don't have dates on the category axis of your chart.
The time-scale axis is available only on 2-D or 3-D line, column, bar, area, or stock chart types.
Make sure the dates are on the category axis of your chart. If they are not, you can change the way data is plotted in a chart.
I'm using dates on my chart, but I don't get a time-scale axis.