The date in excel is actually a number converted to a date. The dates start 1/1/1900.
From memory, I don't have excel at home now to test this, you can run the =IF statement based on the number represented by the date.
=IF(A1<"date number",Period pre2000,IF(- and so on.
Try opening a separate sheet and typing in a date say 1/1/2000 and then highlight/format and choose number. That will give you your number for pre 2000.