The square slider in the timeline can be moved back and forth to jump to a specific month while the photo browser still displays your entire collection. If you want to restrict the browser to a smaller range of photos, you can do so using the left and right end markers. If you just want to see photos from Decemeber of 2003 through February of 2004, you would drag the two end markers to enclose those months. The status bar changes to show how many items fall in that date range and how many items are not shown. When you no longer want to restrict your view to a date range, go to Find > Clear Date Range or press Ctrl-Shift-F. It's a lot faster than dragging the markers in the timeline.
The timeline might be too wide to show every year for which you have photos in your catalog. When it is, the arrows on either end of the timeline will be blue rather than grey. You can click them to scroll the timeline.
If you've changed the photo browser arrangment to folder view, the bars in the timeline change to represent folders, and in import batch view the bars represent import batches.
