I'm not sure this is correct.  It looks like timestamp is in millis.  so `(System.nanoTime()/1000) - timestamp)` may be more correct.

Two things: 1) If I'm wrong, and we are using nano stamps to mark a file to indicate its age, we shouldn't do that.  2) if we're just concerned about getting the age of the file, `System.currentTimeMillis()` is good enough.

Nanos would be needed if we were getting multiple files per millisecond. Even then, nanos wouldn't give a good indication of age--millis-since-epoch would still need to be part of the file name.