When dates are imported to Flatfile, are they automatically reformatted into the US (dd/mm/yyyy) style?

By default, yes. However, you can define a dateFormat field in your config file in Portal 2.0 to format dates any way you like. Use the below example to adjust your date formats:

dateFormat: 'xx/xx/xxxx'

In Portal 3.0 or Workspaces, the smartDateField is available in the Flatfile Platform SDK to enable you to specify your desired date format and ensure your dates are imported in the correct format, regardless of how they are imported.