The Excel Date System
Microsoft Excel saves dates as continuous numbers so that they can be used in calculations. By default, Excel determines a date based on continuous numbers starting at 1, which is January 1st, 1900, and ending with the number 2,958,465, which is December 31st, 9999. This means that Excel can calculate date values only between these two dates.
Microsoft Excel for Mac uses a different date system. The calendar starts at 01/01/1904. For compatibility reasons, the Windows version of Excel provides an option for working with the date system starting at 1904. Select this option only if you have to change worksheets between Windows and Mac computers. But be careful: This setting applies to the active worksheet, and the dates ...