Collects all :training: subtrees from denote.el managed .org files from the past 30 days

denote-filter-training-tagges-headings.el

(defun ct/denote--date-key-regexp (file-type)
  (if (equal file-type 'org)
      "#\\+date:"
    (message "Supports only 'org' at the moment")))

(ct/denote-extract-date-from-front-matter
(defun ct/denote-extract-date-from-front-matter (date-string)
  "Returns a decoded time structure.

Format:
(seconds minutes hour day month year dow dst utcoff)"
  (parse-time-string date-string))

(defun ct/denote--date-value-reverse-function (file-type)
  (if (equal file-type 'org)
      #'ct/denote-extract-date-from-front-matter
    (message "Supports only 'org' at the moment")))

(defun ct/denote-retrieve-front-matter-date (file file-type)
  (denote--file-with-temp-buffer file
    (when (re-search-forward (ct/denote--date-key-regexp file-type) nil t 1)
      (funcall (ct/denote--date-value-reverse-function file-type)
               ;; Assuming the regexp applies to the whole line
               (buffer-substring-no-properties (point) (line-end-position))))))


;; --------------------------------------------------------------------

(let* ((files-and-dates
        (mapcar (lambda (file)
                  (cons file (ct/denote-retrieve-front-matter-date file 'org)))
                (denote-directory-files "_journal")))

       ;; Threshold for how many days back from today to look.
       (days-past 30)

       (files-in-range (seq-keep (lambda (file-with-date)
                                   (let* ((file (car file-with-date))
                                          (date (cdr file-with-date))
                                          (time (encode-time date))
                                          (threshold-date (time-subtract (current-time) (seconds-to-time (* days-past 24 60 60)))))
                                     (if (time-less-p time threshold-date)
                                         nil
                                       file)))
                                 files-and-dates))
       (export (get-buffer-create "*denote export*")))
  (switch-to-buffer-other-window export)
  (erase-buffer)
  (org-mode)
  (mapcar (lambda (file)
            (denote--file-with-temp-buffer file
              (while (outline-next-heading)
                (when (member "training" (org-get-tags))
                  (let (beg end match)
                    (setq beg (point))
                    (org-end-of-subtree t nil)
                    (setq end (point))
                    (setq match (buffer-substring-no-properties beg end))
                    (save-window-excursion
                      (switch-to-buffer export)
                      (insert match "\n")))))))
          files-in-range))