A Microsoft Tech Support document, entitled "XL98: Values Change When You Save in the WK3 or WK4 Format" (dated April 8), notes that "when you open a file that has been saved in Microsoft Excel 98 Macintosh Edition, some of the numbers and dates that appear in the spreadsheet may be incorrect."
The problem can occur if you open a file that had previously be saved as a Lotus 1-2-3 file, such as .WK4 format. It is also necessary that the file contains numbers that are greater than 16,385 -or- The file contains dates that are later than November 9, 1944. "This problem does not occur with numbers and dates that are returned by formulas." Microsoft is working on a fix.
Jeff Bagby, who first reported this to MacFixIt, notes more specifics of the symptoms: "Dollar values entered as $930.78 became $935.78. Dates of 3/17/1998 became 7/17/2076. We have seen this with numbers much lower than the 16,385 that Microsoft claims."