Hello,

The following lines in `org-add-planning-info` (Lines 10844 & 10845 as of commit cc3df3a) do not respect the usage of the org-scheduled-string or org-deadline-string values when prompting the user to enter a schedule or deadline timestamp leading to confusion amongst the inconsistent UI/UX when those strings are changed (as Org Mode permits).

Source/Attribution Credits: Lines 10844 ~ 10845
```
(defun org-add-planning-info (what &optional time &rest remove)
;; Omitted for brevity...
;; If necessary, get the time from the user
(or time (org-read-date nil 'to-time nil
(cl-case what
(deadline "DEADLINE")
(scheduled "SCHEDULED")
(otherwise nil))
default-time default-input)))))
```

Recommend changing to

 ```
(defun org-add-planning-info (what &optional time &rest remove)
;; Omitted for brevity...
;; If necessary, get the time from the user
(or time (org-read-date nil 'to-time nil
(cl-case what
(deadline org-deadline-string)
(scheduled org-scheduled-string)
(otherwise nil))
default-time default-input)))))
```

Thanks,
Matt M.