I lose 4 years (+ 1 day) when syncing Excel files

Can't figure out why, when syncing, all the dates in Excel Mobile files on my AT&T Tilt smartphone lose exactly 4 years and 1 day. If I correct them on the Tilt, then sync, I get 4 years and a day added to the version of the Excel file on my Mac Book. The system dates are right on both units so I can't figure out where the source of the confusion lies. Any thoughts??

Many thanks, technologist. How silly of me to assume that MS and Apple would use a common interpretation of the calendar.