In my agenda I like to see the free blocks of time. As it was suggested in this thread I tried the approach with org-agenda-time-grid. The only problem with that was that the time-grid shows up even when the time is busy. By doing some research I found a solution on this page (Org ad hoc code, quick hacks and workarounds):
(defun org-time-to-minutes (time) "Convert an HHMM time to minutes" (+ (* (/ time 100) 60) (% time 100))) (defun org-time-from-minutes (minutes) "Convert a number of minutes to an HHMM time" (+ (* (/ minutes 60) 100) (% minutes 60))) (defadvice org-agenda-add-time-grid-maybe (around mde-org-agenda-grid-tweakify (list ndays todayp)) (if (member 'remove-match (car org-agenda-time-grid)) (flet ((extract-window (line) (let ((start (get-text-property 1 'time-of-day line)) (dur (get-text-property 1 'duration line))) (cond ((and start dur) (cons start (org-time-from-minutes (+ dur (org-time-to-minutes start))))) (start start) (t nil))))) (let* ((windows (delq nil (mapcar 'extract-window list))) (org-agenda-time-grid (list (car org-agenda-time-grid) (cadr org-agenda-time-grid) (remove-if (lambda (time) (find-if (lambda (w) (if (numberp w) (equal w time) (and (>= time (car w)) (< time (cdr w))))) windows)) (caddr org-agenda-time-grid))))) ad-do-it)) ad-do-it)) (ad-activate 'org-agenda-add-time-grid-maybe)
This has worked great until the most recent update to org-mode was released (9.1). There is a new org duration library (org-duration.el) and with its introduction several functions became obsolete which broke the above mentioned defadvice.
To my (amateurish) unterstanding the problem is that the org duration library works with strings which also contain colons (e.g. 19:30 instead of 1930). This is why I have tried to change the functions org-time-to-minutes and org-time-from-minutes to output strings with colons as well. This did not work, however, which is why I am hoping for the community to find a fix.
(Edit: To be honest my ideal solution would not remove the grid lines which are in the range of an appointment but rather change them in a way that there is a visual time block, e.g. by changing the color of the grid lines. But since this is probably much more difficult to achieve, I would already be happy with a fix of the defadvice.)