Calculating dates prior to Jan 1, 1900?

If you had to use Excel to calculate George Washington's birthday from his birthdate to today...you can't do it because Excel's internal date function only works from Jan 1, 1900. But I do remember there's a free add on that allows Excel to go beyond this and calculate dates prior to Jan 1, 1900. Does anybody remember that add on? Or does anybody know of a way to calculate dates prior to the 1900 limit? Thanks in advance.