Date format issue

I am importing a large excel file from with ~2500 line items. I am having issues with excel formating some of the dates to a year 19xx format instead of a 20xx format.

For example: some of the values that are being copied are 3/15/43 or 9/22/47. Then value is being converted to 03/15/1943 and to 09/22/1947. Is there an easy way to convert these value back to a 20xx format. I tried using the custom button with "mm/dd/20yy" but didn't work. Thanks for any comments.