Because Microsoft Excel does not recognize dates before 1/1/1900, if you
enter a formula in which the year argument of the DATE function is less
than 1900, Microsoft Excel adds 1900 to the year argument. The following example illustrates this behavior:
Because the year argument (98) is less than 1900, Microsoft Excel adds
1900 to the argument (1900 + 98 = 1998) and returns the following date:
Because versions of Microsoft Excel earlier than Excel 97 cannot handle dates greater than 12/31/2078, if the value of the year argument becomes larger than 2078, the DATE function returns a #NUM! error value. This is normal behavior for earlier versions of Microsoft Excel.
However, because Microsoft Excel 97 and later can handle dates through the year 9999, you do not receive a #NUM! error value unless the value of the year argument is greater than 9999.