Whilst it would be possible to write a macro for each application (i.e. one for Word, another for Excel, another for PowerPoint, etc) to pre-populate the application 'Save As' dialogues with a string such as you require, it would be much more difficult to prevent a user ignoring the rules and giving the file an inappropriate name (e.g. saving what should be "2013-11_Sales-report_for_client" as "2014-01_Sales-report_for_client" or "2012-12_Sales-report_for_client"). After all, just as you might usually want the files to include the current year & month, I'm sure there are cases where the current year & month aren't appropriate.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]