Right now (modulo bug #657186) the recent-files mode in the file chooser displays a plain list of files/folders, sorted by mtime:

   foo.txt            2K    08:50
   bar.pdf          200K    yesterday
   baz.jpg          1.2M    01/01/2011

When there are many items in the list, it would be useful to have headings that separate each chunk of dates.  For example

   Today
     foo.txt            2K    08:50
     bar.txt            1K    07:25
   Last week
     bar.pdf          200K    yesterday
     ...
   January 2011
     baz.jpg          1.2M    01/01/2011

Etc.  We already have a "chunking" algorithm for timestamps in the journal extension for gnome-shell: look for "bucket" inside https://github.com/kitallis/gnome-shell-kitallis/blob/journal/js/ui/journalDisplay.js

We can do this for the file chooser with a custom treemodel that wraps the GtkFileSystemModel used for recent-files, perhaps.  The headings would be unselectable rows.  That wrapper model would only be used when the "modified" column in the list is being used for sorting.