org-capture: Add support for month trees

* doc/org-manual.org: Add `:tree-type month' option for capture
  templates.
* lisp/org-capture.el (org-capture-set-target-location): Add
  `:tree-type month' option to capture templates to group entries by
  month.
* lisp/org-datetree.el (org-datetree-find-month-create): Add
  `org-datetree-find-month-create' function to add datetree entries
  grouped by month.
* testing/lisp/test-org-datetree.el
  (test-org-datetree/find-month-create): Add test for new function.

doc/org-manual.org

@@ -7601,9 +7601,10 @@ Now lets look at the elements of a template definition.  Each entry in
 
   - ~:tree-type~ ::
 
-    When ~week~, make a week tree instead of the month tree, i.e.,
-    place the headings for each day under a heading with the current
-    ISO week.
+    Use ~week~ to make a week tree instead of the month-day tree,
+    i.e., place the headings for each day under a heading with the
+    current ISO week.  Use @code{month} to group entries by month
+    only.  Default is to group entries by day.
 
   - ~:unnarrowed~ ::
 

etc/ORG-NEWS

@@ -392,7 +392,9 @@ the headline to use for making the table of contents.
 ,* Another section
 ,#+TOC: headlines 1 :target "#TargetSection"
 #+end_example
-
+*** New option to group captured datetime entries by month
+A new `:tree-type month' option was added to org-capture-templates to
+group new datetime entries by month.
 ** New functions
 *** ~org-dynamic-block-insert-dblock~
 
@@ -405,6 +407,7 @@ dynamic block in ~org-dynamic-block-alist~.
 *** ~org-table-cell-left~
 *** ~org-table-cell-right~
 *** ~org-habit-toggle-display-in-agenda~
+*** ~org-datetree-find-month-create~
 ** Removed functions and variables
 *** Removed Org Drill
 

lisp/org-capture.el

@@ -998,9 +998,10 @@ Store them in the capture property list."
 	   ;; Make a date/week tree entry, with the current date (or
 	   ;; yesterday, if we are extending dates for a couple of hours)
 	   (funcall
-	    (if (eq (org-capture-get :tree-type) 'week)
-		#'org-datetree-find-iso-week-create
-	      #'org-datetree-find-date-create)
+	    (pcase (org-capture-get :tree-type)
+	      ('week #'org-datetree-find-iso-week-create)
+	      ('month #'org-datetree-find-month-create)
+	      (t #'org-datetree-find-date-create))
 	    (calendar-gregorian-from-absolute
 	     (cond
 	      (org-overriding-default-time

lisp/org-datetree.el

@@ -51,11 +51,29 @@ Added time stamp is active unless value is `inactive'."
 
 ;;;###autoload
 (defun org-datetree-find-date-create (d &optional keep-restriction)
-  "Find or create an entry for date D.
+  "Find or create a day entry for date D.
 If KEEP-RESTRICTION is non-nil, do not widen the buffer.
 When it is nil, the buffer will be widened to make sure an existing date
 tree can be found.  If it is the symbol `subtree-at-point', then the tree
 will be built under the headline at point."
+  (org-datetree--find-create-group d 'day keep-restriction))
+
+;;;###autoload
+(defun org-datetree-find-month-create (d &optional keep-restriction)
+  "Find or create a month entry for date D.
+Compared to `org-datetree-find-date-create' this function creates
+entries grouped by month instead of days.
+If KEEP-RESTRICTION is non-nil, do not widen the buffer.
+When it is nil, the buffer will be widened to make sure an existing date
+tree can be found.  If it is the symbol `subtree-at-point', then the tree
+will be built under the headline at point."
+  (org-datetree--find-create-group d 'month keep-restriction))
+
+(defun org-datetree--find-create-group
+    (d time-grouping &optional keep-restriction)
+  "Find or create an entry for date D.
+If time-period is day, group entries by day. If time-period is
+month, then group entries by month."
   (setq-local org-datetree-base-level 1)
   (save-restriction
     (if (eq keep-restriction 'subtree-at-point)
@@ -84,9 +102,10 @@ will be built under the headline at point."
       (org-datetree--find-create
        "^\\*+[ \t]+%d-\\([01][0-9]\\) \\w+$"
        year month)
-      (org-datetree--find-create
-       "^\\*+[ \t]+%d-%02d-\\([0123][0-9]\\) \\w+$"
-       year month day))))
+      (when (eq time-grouping 'day)
+	(org-datetree--find-create
+	 "^\\*+[ \t]+%d-%02d-\\([0123][0-9]\\) \\w+$"
+	 year month day)))))
 
 ;;;###autoload
 (defun org-datetree-find-iso-week-create (d &optional keep-restriction)

testing/lisp/test-org-datetree.el

@@ -113,6 +113,17 @@
 	(org-datetree-find-date-create '(3 29 2012)))
       (buffer-substring (point) (line-end-position))))))
 
+(ert-deftest test-org-datetree/find-month-create ()
+  "Test `org-datetree-find-month-create' specifications."
+  ;; When date is missing, create it with the entry under month.
+  (should
+   (string-match
+    "\\`\\* 2012\n\\*\\* 2012-03 .*\\'"
+    (org-test-with-temp-text ""
+      (let ((org-datetree-add-timestamp nil))
+	(org-datetree-find-month-create '(3 29 2012)))
+      (org-trim (buffer-string))))))
+
 (ert-deftest test-org-datetree/find-iso-week-create ()
   "Test `org-datetree-find-iso-date-create' specificaiton."
   ;; When date is missing, create it.