Standard dates are based on December 30, 1899 and are considered "Standard 1900" dates.
BackwardsCompatibleBase1900
1
Excel for Windows backwards compatible dates are based on December 31,
1899 are considered "Backwards compatible 1900" dates.
BackwardsCompatibleBase1904
2
Excel for Macintosh backwards compatible dates are based on January 1,
1904 and are considered "1904" dates.
Remarks
Information on date serial conversion is available here:
http://www.documentinteropinitiative.com/implnotes/ISO-IEC29500-2008/001.018.017.004.001.000.000.aspx