Initally when you created the panel on your dashboard there is a "Time Range" portion of the creation process where you can select the "Shared Time Picker" there.
If you already created the panel, go into edit mode for the dashboard to edit the different panels. Click the magnifying glass on the panel and a box will open where you can select the time range for the panel.
Keep in mind this will only affect the panel that you clicked on.
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