1900 or 1904?

I just went through some fun wall banging trying to figure out why copying and pasting a date across two spreadsheets ended up with different dates? WTF is that?

In other words I would copy '09/22/1979' and end up with '09/01/1975' .. or something like that.

Well ... Excel can calculate the date based upon 1900 or 1904. This is set in advanced excel options for each workbook/file. Here is the MS support doc:

http://support.microsoft.com/kb/180162

Let me copy/paste the main problem:

By default, Microsoft Excel for the Macintosh uses the 1904 date system. Because of the design of early Macintosh computers, dates before January 1, 1904 were not supported; this design was intended to prevent problems related to the fact that 1900 was not a leap year. Note that if you switch to the 1900 date system, Microsoft Excel for the Macintosh does support dates as early as January 1, 1900. 

So as I was switching between Excel in Mac and Excel in Windows because Office for the Mac is a piece of sh*t and because I wanted to use more complex features ... I ran into date problems.

Edit:
Okay... I want to be clear on a few things.

1. The setting is saved in the Excel workbook .... So sending files back and forth across Windows/Mac and with other people is okay. It will keep the setting and calculate the date correctly. 
2. The issue only occurs if you are copying/pasting dates across different files (Excel Workbooks) where the setting is different in each file. 

Lastly, The MS support doc has more detail and some tips/tricks on working between files.