I have a date slicer which should show last 12 days as selected when report opens. Further, user should have flexibility to select any other date range.
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Can you try Relative Date option for the date slicer as shown below image-

In this case, you can select last 12 Days in the slicer while publish. User can then switch to any other date range as per necessity. But while initial loading, report will always show last 12 days in the selection.
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