[BUG] org-agenda: incorrect display of multi-day date ranges [9.6.15 (release_9.6.15 @ /usr/share/emacs/29.3/lisp/org/)]

[hrdl]

Parts of org-agenda do not handle time ranges spanning multiple days
correctly. This is exarcabated when setting
org-agenda-default-appointment-duration , which adds an incorrect end
time to the entries corresponding to the start and end date. To
reproduce:

(prog
  (find-file "test.org")
  (insert "* Entry\n<2024-05-30 Thu 14:00>--<2024-05-31 Fri 16:00>")
  (save-buffer)
  (setq org-agenda-default-appointment-duration 30) ;; optional
  (org-agenda-file-to-front)
  (org-agenda-list)
  ;; Or, for ease of debugging
  (with-current-buffer "test.org"
    (setq date '(5 30 2024)) ;; Or 31
    (org-agenda-get-blocks))
  )

At the moment I use the following workaround, which would benefit from some 
feedback.

>From 48192b14d3d5739467df7c41ac4e1e9c94f81ee4 Mon Sep 17 00:00:00 2001
From: hrdl <git@hrdl.eu>
Date: Wed, 29 May 2024 17:43:53 +0200
Subject: [PATCH] org-agenda: handle multi-day time ranges

* lisp/org-agenda.el: (org-agenda-format-item, org-agenda-get-blocks):
Handle start and end dates of multi-day time ranges
---
 lisp/org-agenda.el | 24 +++++++++++++++---------
 1 file changed, 15 insertions(+), 9 deletions(-)

diff --git a/lisp/org-agenda.el b/lisp/org-agenda.el
index 10f25be8a..2e4c01e50 100644
--- a/lisp/org-agenda.el
+++ b/lisp/org-agenda.el
@@ -6871,9 +6871,9 @@ scheduled items with an hour specification like [h]h:mm."
                               ((and (= d1 d0) (= d2 d0))
                               (concat "<" start-time ">--<" end-time ">"))
                               ((= d1 d0)
-                              (concat "<" start-time ">"))
+                              (concat "<" start-time ">--"))
                              ((= d2 d0)
-                              (concat "<" end-time ">")))
+                              (concat "--<" end-time ">")))
                             remove-re))))
              (org-add-props txt props
                 'face face
@@ -7013,21 +7013,24 @@ Any match of REMOVE-RE will be removed from TXT."
           ;; Normalize the time(s) to 24 hour.
          (when s1 (setq s1 (org-get-time-of-day s1 t)))
          (when s2 (setq s2 (org-get-time-of-day s2 t)))
+          ;; Possibly swap time(s) if this is an end timestamp of a multi-date 
time span
+          (when (and s1 (not s2) (string-prefix-p "--" dotime))
+            (cl-rotatef s1 s2))
          ;; Try to set s2 if s1 and
          ;; `org-agenda-default-appointment-duration' are set
-         (when (and s1 (not s2) org-agenda-default-appointment-duration)
+         (when (and s1 (not s2) (not (string-suffix-p "--" dotime)) 
org-agenda-default-appointment-duration)
            (setq s2
                  (org-duration-from-minutes
                   (+ (org-duration-to-minutes s1 t)
                      org-agenda-default-appointment-duration)
                   nil t)))
          ;; Compute the duration
-         (when s2
+         (when (and s1 s2)
            (setq duration (- (org-duration-to-minutes s2)
                              (org-duration-to-minutes s1))))
           ;; Format S1 and S2 for display.
          (when s1 (setq s1 (format "%5s" (org-get-time-of-day s1 'overtime))))
-         (when s2 (setq s2 (org-get-time-of-day s2 'overtime))))
+         (when s2 (setq s2 (format "%5s" (org-get-time-of-day s2 'overtime)))))
        (when (string-match org-tag-group-re txt)
          ;; Tags are in the string
          (if (or (eq org-agenda-remove-tags t)
@@ -7065,14 +7068,17 @@ Any match of REMOVE-RE will be removed from TXT."
                              "")))
                    (if (equal "" s) "" (concat s 
org-agenda-breadcrumbs-separator))))))
        (setq time (cond (s2 (concat
-                             (org-agenda-time-of-day-to-ampm-maybe s1)
+                              (if s1 (org-agenda-time-of-day-to-ampm-maybe s1)
+                                (make-string (if org-agenda-timegrid-use-ampm 
7 5) ? ))
                              "-" (org-agenda-time-of-day-to-ampm-maybe s2)
                              (when org-agenda-timegrid-use-ampm " ")))
                         (s1 (concat
                              (org-agenda-time-of-day-to-ampm-maybe s1)
-                             (if org-agenda-timegrid-use-ampm
-                                  (concat time-grid-trailing-characters " ")
-                                time-grid-trailing-characters)))
+                              (if (string-suffix-p "--" dotime)
+                                  (concat "-" (make-string (if 
org-agenda-timegrid-use-ampm 7 5) ? ))
+                               (if org-agenda-timegrid-use-ampm
+                                    (concat time-grid-trailing-characters " ")
+                                  time-grid-trailing-characters))))
                         (t ""))
              category (if (symbolp category) (symbol-name category) category)
              level (or with-level ""))
-- 
2.45.1

Emacs  : GNU Emacs 29.3 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.41, 
cairo version 1.18.0)
Package: Org mode version 9.6.15 (release_9.6.15 @ 
/usr/share/emacs/29.3/lisp/org/)