When you type a date into a cell, Excel probably displays it with a two-digit year, such as 15/8/99. If you want all new dates you enter to have four-digit years (a good move for Y2K readiness), you must change the default date format.
Excel uses Windows' system-wide setting as its default. To change this, select Start-Settings, Control Panel-Regional Settings.
Click the Date tab and choose a different format from the "Short date" style list. Date styles that contain yyyy will display four-digit years.