"Last Modified" dates are not ideal for finding most recently added music. If you install Playback Statistics it has an %added% field which records the date/time music was added to library, and is not changing when you modify files. The downside is if you only newly installed it, everything gets the same added date (current date), so not immediately useful until you add new albums.
In title formatting (library view patterns), to get descending dates you subtract the dates like Air KEN was attempting. Library Tree has a custom $nodisplay function for sorting views, so the view pattern should look like -
$nodisplay{$sub(99999999999999,$replace($date(%last_modified%),-,))}$date(%last_modified%)|%filename_ext%
^ This only sorts by the $date part of %last_modified%, which means tracks will be grouped by day, rather than the precise minutes and seconds they were modified which would most likely group everything separately. (If you use Playback Statistics you can swap %last_modified% for %added% in both places above)
^ For secondary branch %filename_ext%, keep in mind Library Tree sorts the playlist by the complete view pattern, so depending on your filename structure you might need more fields to keep the playlist organized (or just use standard tags here like %discnumber% etc, or put them in another $nodisplay)
Subtracted values do not display or can't be successfully replaced by fields.
It was not working because $sub function only accepts numbers, but %added% field (and other dates) have a dash, space, and colon which also need replacing -
$sub(99999999999999,$replace(%last_modified%,-,, ,,:,))