If you are using the templates for MDX calculation built into SSAS 2008 you might run into some strange issues. In the image below you can see the calculation is working for all years except for 2008.

My first thought was data issue. After querying the table and finding no issues I returned to the MDX calculations in the cube and started trouble shooting. Here is what I found. The format string is set to standard when you drag over the Period to Date template.

But the word standard does not have double quotes around it. If you click on the drop down menu and select standard it has the double quotes around it.

This fixed my problem.