One of the ways Microsoft Excel stores dates is as serial numbers which are formatted to be used in calculations (more here). This means that by default, January 1, 1900 is serial number 1, with each new day being assigned the next sequential number. 

If you've imported serial dates from Excel into Airtable you can use a DATE_ADD() function in your table to add the appropriate amount of days (the number imported from your Excel file) starting from 1/1/1900. 

DATEADD('1/1/1900',({Serial Date Number}-1),'days')

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​As a note, you need to ensure that the option to use GMT for all collaborators is toggled off in the formula field settings.