Excel supports two data systems: 1900 and 1904. Early Macintoshes did not support dates before Jan. 1, 1904. For this reason, Excel for Macs renders entered dates as the number of days since 1/1/1904. Excel for Windows, on the other hand, supports dates starting Jan. 1, 1900, and so renders dates as the number of days since 1/1/1900.

For example, the date 1/1/1904 is represented as a 0 in the 1904 date system. The same date in the 1900 date system is represented by the number 1,462. This is because 1/1/1904 is 1,462 days since 1/1/1900, the day the 1900 date system begins its “counting.”

What all this means to you is that if you have users on both Macs and PCs using Excel and copying data between workbooks, there’s the chance of incorrect date representation.
One solution is to set your PCs so that they all use the 1904 date system. Then they’ll be compatible with the Macs. To do so, open a workbook and select Tools | Options. Click the Calculation tab. Under Workbook Option, select the 1904 Date System check box and click OK. For more information, check out Microsoft’s Knowledge Base article 214330, which covers this issue in depth:
http://support.microsoft.com/support/kb/articles/Q214/3/30.asp