Year keeps on changing in date when I open an excel worksheet that was emailed

I think one of the machines is set up to use the Apple compatible 1904 date system. Windows counts days from 1st January 1900, but owing to an error in VisiCalc, the first spreadsheet released in 1979, February 1900 was deemed to be a leap year when in fact it was not, and this can lead to errors in the first 60 days of 1900: the Weekdays and Dates don't match! However, by the time this was discovered, it was deemed too late to fix, so the error has been perpetuated. When the Mac was released, Apple decided to avoid the issue by starting their count on 2nd January 1904. If you are running a Mac and the sender is on a PC, this might account for the problem. Set Excel on your Mac to use the 1900 system (which will affect all dates in your existing workbooks) - or work out the days difference and subtract that from the dates in the received workbook!