Why does Powerpoint for Mac not preserve filenames with a slash (/)? I am designing lectures in Powerpoint (the latest released version this year) for a class I'm teaching soon, and created filenames based on the date that I will give them:



But if I open 9/22.pptx (or any one of the other files) in Powerpoint, it does not preserve the filename:



(See that the 9/ part was removed.)

Is there a reason for this? If I open the presentation in Keynote (the latest version), the name is preserved:
From Microsoft's Known Issues with PowerPoint 2011:


  Some characters cannot be used in file and folder names in PowerPoint
  for Mac 2011
  
  IMPORTANT   This issue is fixed in Office for Mac 2011 Service Pack 1.
  Click Downloads to download Service Pack 1 or the latest update.
  
  Characters, including &lt;"> and |, are allowed in file names in
  PowerPoint 2008 and earlier versions, but they must be removed from a
  file name before it can be opened in PowerPoint 2011.


Check your version of PowerPoint for updates to see if this addresses or fixes the issue.

Otherwise, change the filename to include a character that is allowed such as "-". (e.g.: 9-22)