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Excel for Windows uses the 1900 date system by default, Macintosh uses the 1904 date system by default. If you are using both, switch the options on one of them so they match.
From the help file:
Microsoft Excel stores dates as sequential numbers which are called serial values. By default, January 1, 1900 is serial number 1, and January 1, 2008 is serial number 39448 because it is 39,448 days after January 1, 1900. Excel stores times as decimal fractions because time is considered a portion of a day.
Because dates and times are values, they can be added, subtracted, and included in other calculations. You can view a date as a serial value and a time as a decimal fraction by changing the format of the cell that contains the date or time to General format.
Because the rules that govern the way that any calculation program interprets dates are complex, you should be as specific as possible about dates whenever you enter them. This will produce the highest level of accuracy in your date calculations.
The 1900 and 1904 date systems
Excel supports two date systems: the 1900 and 1904 date systems. The default date system for Microsoft Excel for Windows is 1900. The default date system for Microsoft Excel for the Macintosh is 1904. You can change the date system. On the Tools menu, click Options, click the Calculation tab, and then select or clear the 1904 date system check box.
The date system is changed automatically when you open a document from another platform. For example, if you are working in Excel for Windows and you open a document created in Excel for the Macintosh, the 1904 date system check box is selected automatically.
The following table shows the first date and the last date for each date system and the serial value associated with each date.
Date system First date Last date
1900 January 1, 1900
(serial value 1) December 31, 9999
(serial value 2958465)
1904 January 2, 1904
(serial value 1) December 31, 9999
(serial value 2957003)
