It looks like the correction of the day name in timestamps is hardcoded in `org-ctrl-c-ctrl-c':
((or `timestamp (and `planning (guard (org-at-timestamp-p
'lax))))
(org-timestamp-change 0 'day))
`org-clock-update-time-maybe' uses a regexp to match the start and end times and hands those off to `org-parse-time-string' which does not consider the day name.
