Dates are dependent on the OS settings that were current when the file was created and on the formatting of it for display.
If you are in the USA and have the correct localisation settings for your region (which is the default) then the date formats you will use when a database is created will be in the m/d/Y format.
If you open a file created in Australia you will be challenged when you open the file to use the files date formats or the system ones. We enter dates in d/m/Y format.
As your sample has commas... I assume you either have opened a database on a machine with European settings or a file created in Europe... OR you have changed the formatting to display in that format.... OR you have elected to use the formats saved in the file.
Whilst you can change the formatting to display dates in various formats, not all of those formats are valid for entry. Many of those formats Excel accepts are not valid dates in FMP.
To fix things...:
Check your OS date formats are correct for your region.
Open the databases and save as a clone (no records)
Import your data into the new databases.
Obviously if the data itself is in a format that cannot be read, you might have to import it into a new temp field so that you can recalculated it back to the correct format replacing the dates in the date field.
Message was edited by: Lyndsay Howarth because I misspelled my name ;-)