I suspect you're encountering an old bug with Excel's pivot tables that causes the column widths to be reset incorrectly when a pivot table refreshes. You can prevent this from happening by going into the Pivot Table Options window and unticking Autofit column widths on update.
You can see a walkthrough of this in the video lesson: Expert Skills Lesson 5-25 Use slicers to filter multiple pivot tables.
You can also see a Preserve cell formatting on update option in this window. Keeping this enabled should prevent your date formats from being changed after a refresh.
If you've been setting the pivot table formats by formatting cells instead of using the pivot table field settings this could also account for the problem, since the pivot table will default to its own number format when adding new rows or columns.
You can see how to set the pivot table field settings in the video tutorial: Expert Skills Lesson 5-1 Create a pivot table.
I hope these are the answers you were looking for, but please feel free to reply if you are still having problems.
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