Select any empty cell.
Select your dates
Operation...Add - you should get a bunch of 5 digit numbers
Format cells as Date
Sometimes dates that are imported from external sources get stuck as text and will not let that format go no matter how hard you try. By "adding" a blank cell, you are essentially adding "0 days" to the text-dates. This forces Excel to recognize the contents as the number that Excel uses to internally store the date. Once it has been converted to it's internal "serial number" (the 5 digit number) that value can be formatted as a date.
See here to read about how Excel stores Dates and Times internally:
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