Excel supports two date systems, the 1900 date system and the 1904 date system. Each date system uses a unique starting date from which all other workbook dates are calculated. All versions of Excel for Windows calculate dates based on the 1900 date system. Excel 2008 for Mac and earlier Excel for Mac versions calculate dates based on the 1904 date system. Excel 2016 for Mac and Excel for Mac 2011 use the 1900 date system, which guarantees date compatibility with Excel for Windows.
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. If this option is disabled, you will receive a message asking whether the dates should be converted when pasted. You have two options. You can convert the dates to use the 1900 date system (recommended). This option makes the dates compatible with other dates in the workbook. Or, you can keep the 1904 date system for the pasted dates only.
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. You must convert these dates manually.
The 1900 date system
In the 1900 date system, dates are calculated by using January 1, 1900, as a starting point. When you enter a date, it is converted into a serial number that represents the number of days elapsed since January 1, 1900. For example, if you enter July 5, 2011, Excel converts the date to the serial number 40729. This is the default date system in Excel for Windows, Excel 2016 for Mac, and Excel for Mac 2011. If you choose to convert the pasted data, Excel adjusts the underlying values, and the pasted dates match the dates that you copied.
The 1904 date system
In the 1904 date system, dates are calculated by using January 1, 1904, as a starting point. When you enter a date, it is converted into a serial number that represents the number of days elapsed since January 1, 1904. For example, if you enter July 5, 2011, Excel converts the date to the serial number 39267. This is the default date system in earlier versions of Excel for Mac. If you choose not to convert the data and keep the 1904 date system, the pasted dates vary from the dates that you copied.
The difference between the date systems
Because the two date systems use different starting days, the same date is represented by different serial numbers in each date system. For example, July 5, 2011, can have two different serial numbers, as follows:
The difference between the two date systems is 1,462 days. This means that the serial number of a date in the 1900 date system is always 1,462 days greater than the serial number of the same date in the 1904 date system. 1,462 days is equal to four years and one day (including one leap day).