I would recommend that you use text so that they will not change; however,
if you want to risk using a macro John Walkenbach wrote
Extended Dates routines
Excel does not recognize dates before Jan 1, 1900 and mishandles leap year in 1900
so for simply calculating date differences (age) you might use John Walkenbach's
XDATEDIF Extended Date Functions Add-In, instead of DATEDIF, eliminating problems
with negative dates involving subtraction in MS date system and incorrect leap years in
older MS 1900 date system. http://www.j-walk.com/ss/excel/files/xdate.htm
You calculations can be further complicated by when a country switched calendars.
Excel's normally formatted dates only go back that far. You can use custom
add-ins or just use Julian dates ( without fancy formatting and support
functions.