Currently, I use a `YYMMDD-NAME+PAGE` name for most of my files. `NAME` has spaces converted to underscores.

I'd like to use the `YYYY-MM-DD` date format, but I am not sure how to separate it from the name. A `-` would look strange if the name started with a number. If I use a `_`, then it conflicts with the underscore representing a space.

What characters are reasonably safe in file names that would work here? I am on Linux, but I might share files with other people (Windows 7, Mac OS X).