Hi Dirk,
In the past I have done this into a separate formula text field by using the SearchAndReplace function to search a field for any instance where a - appears and replacing it with a /. That way when you export a report you can specifically export that field and any dates that were captured in the other format will get changed and any dates that are already in the right format should be ignored and passed over correctly.
SearchAndReplace formula reference------------------------------
Evan Martinez
Community Marketing Manager
Quick Base
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