The "ls -l" command will list modified times. Linux has extensions that allow a more useful formatting of that time, specifically the "--time-style' option. It uses the same date/time formatting string syntax that the "date" command does.
So you can get the difference between file time and current time by using the "ls" and "time" commands to output simple numeric values and subtracting them. I'm not sure about how to format that interval in something like "days, months, weeks" or whatever.
Of course, for many problems, this is doing things the hard way. If you just want to archive old files that haven't been used in 6 months (180 days), you can do things like this:
zip -r -m unused-files.zip `find -atime +180`
You can use this with "tar" as well, although I don't think tar has an equivalent to the zip "-m" (move) option. Tar has a built-in date threshold test, but I think it only works on modification times, which doesn't help for finding unused read-only files.
Customer surveys are for companies who didn't pay proper attention to begin with.