In the 1900 date system, dates are calculated by using January 1, 1900, as a starting point.

Note: Dates that are copied as part of a chart cannot be converted from the 1904 date system and will vary by approximately four years. Or, you can keep the 1904 date system for the pasted dates only.

This option makes the dates compatible with other dates in the workbook. You can convert the dates to use the 1900 date system (recommended). If this option is disabled, you will receive a message asking whether the dates should be converted when pasted. When you copy dates from a workbook created in an earlier version to a workbook created in Excel 2016 or 2011 for Mac, they will be converted automatically unless the option to 'Automatically convert date system' is disabled in Preferences > Edit > Date Options. Excel 2016 for Mac and Excel for Mac 2011 use the 1900 date system, which guarantees date compatibility with Excel for Windows. Excel 2008 for Mac and earlier Excel for Mac versions calculate dates based on the 1904 date system. All versions of Excel for Windows calculate dates based on the 1900 date system. Each date system uses a unique starting date from which all other workbook dates are calculated. Excel supports two date systems, the 1900 date system and the 1904 date system.

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