Support sorting files and directories by modification time.

doc/NERDTree.txt

@@ -995,17 +995,33 @@ appended to the array.
 
 The regex '\/$' should be used to match directory nodes.
 
+A special flag can be used to sort by the modification timestamps of files and
+directories. It is either '[[timestamp]]' for ascending, or '[[-timestamp]]'
+for descending. If placed at the beginning of the list, files and directories
+are sorted by timestamp, and then by the remaining items in the sort order
+list. If this flag is in any other position of the list, timestamp sorting is
+done secondarily. See examples 4, 5, and 6 below.
+
 After this sorting is done, the files in each group are sorted alphabetically.
 
 Other examples: >
     (1) ['*', '\/$']
     (2) []
     (3) ['\/$', '\.rb$', '\.php$', '*', '\.swp$',  '\.bak$', '\~$']
+    (4) ['[[timestamp]]']
+    (5) ['\/$', '*', '[[-timestamp]]']
+    (6) ['\.md$', '\.c$', '[[-timestamp]]', '*']
 <
 1. Directories will appear last, everything else will appear above.
 2. Everything will simply appear in alphabetical order.
 3. Dirs will appear first, then ruby and php. Swap files, bak files and vim
    backup files will appear last with everything else preceding them.
+4. All files and directories are sorted by timestamp, oldest first. If any
+   files have identical timestamps, they are sorted alphabetically.
+5. Directories are first, newest to oldest, then everything else, newest to
+   oldest.
+6. Markdown files first, followed by C source files, then everything else.
+   Each group is shown newest to oldest.
 
 ------------------------------------------------------------------------------
                                                         *'NERDTreeStatusline'*

lib/nerdtree/path.vim

@@ -392,21 +392,27 @@ endfunction
 " FUNCTION: Path.getSortKey() {{{1
 " returns a key used in compare function for sorting
 function! s:Path.getSortKey()
-    if !exists("self._sortKey") || g:NERDTreeSortOrder !=# g:NERDTreeOldSortOrder
+    let l:ascending = index(g:NERDTreeSortOrder,'[[timestamp]]')
+    let l:descending = index(g:NERDTreeSortOrder,'[[-timestamp]]')
+    if !exists("self._sortKey") || g:NERDTreeSortOrder !=# g:NERDTreeOldSortOrder || l:ascending >= 0 || l:descending >= 0
+        let self._sortKey = [self.getSortOrderIndex()]
+
+        if l:descending >= 0
+            call insert(self._sortKey, -getftime(self.str()), l:descending == 0 ? 0 : len(self._sortKey))
+        elseif l:ascending >= 0
+            call insert(self._sortKey, getftime(self.str()), l:ascending == 0 ? 0 : len(self._sortKey))
+        endif
+
         let path = self.getLastPathComponent(1)
         if !g:NERDTreeSortHiddenFirst
             let path = substitute(path, '^[._]', '', '')
         endif
         if !g:NERDTreeCaseSensitiveSort
             let path = tolower(path)
         endif
-        if !g:NERDTreeNaturalSort
-            let self._sortKey = [self.getSortOrderIndex(), path]
-        else
-            let self._sortKey = [self.getSortOrderIndex()] + self._splitChunks(path)
-        endif
-    endif
 
+        call extend(self._sortKey, (g:NERDTreeNaturalSort ? self._splitChunks(path) : [path]))
+    endif
     return self._sortKey
 endfunction