Before, each dashboard graph widget used its own date settings. They can still be set to do so, but now they can also be set to "Use Global Dates."
Figure 1: Graph Widget settings. Note the "Use Global Dates" check box
If any graph widgets are set to use the global dates, a date tool will appear at the top of the dashboard to allow global control of dates for those widgets which have that option selected.
Figure 2: The date tool