Date change when copying from Excel 2000

We are e-mailing an Excel 2000 spreadsheet around our company. When someone receives the spreadsheet, they cut out the lines they need and paste them to their own spreadsheet or a new spreadsheet. When they do this, the dates change - i.e. 1/14/02becomes 1/13/98, 12/14/01 becomes 12/13/97. It always subtracts 1 day and 4 years from the original date. Any ideas why this happens?