Once you've selected a specific date upon which to view logged data (see Select a Date Upon Which to View Logged Data on the Historical Data Viewer Page) and have selected an appropriate time duration (see Modify the Time Duration for the Data Plotted on the Historical Data Viewer Page), you may use the scroll bar at the bottom of the Historical Data Viewer page to move backward or forward in time, in the increments you set using the duration drop-down list.
For example, if you've used the Select Start Date button to set November 1, 2003, and have set the duration drop-down list to 6 Hours, the graph will initially display 6-hours worth of data beginning at the current time (as configured under your Windows operating system) on November 1st. As you click the backward scroll bar button, the graph will adjust to display earlier data in 6-hour increments.
You can also drag the scroll button to change the display in any increment.