[BUG] Undocumented convention for org-time-stamp-custom-formats to be "<...>" (was: time-stamp in DONE tag is not really displayed)

[Ihor Radchenko]

Uwe Brauer <oub@mat.ucm.es> writes:

> My time-stamps are of the form <2022-10-23 Sun>
> I have an entry like this 
>
> - State "DONE"       from "WAIT"       [2022-10-23 21:06] \\
>
>
>
> However it is displayed when I use org-toggle-time-stamp-overlays  as
>  [23.10.%]
>
> Neither the year not the time is displayed, why!
>  
> I have set 
> org-time-stamp-custom-formats 
> to 
> (" %d.%m.%Y " . " %d.%m.%Y")
>
> I am puzzled, any ideas?

Confirmed.

This is because Org expects the first and the last characters in
org-time-stamp-custom-formats to be opening/closing brackets.
(undocumented)

Why?
Because org-time-stamp-formats does so.

Why does org-time-stamp-formats does so?
No idea.
This code dates back to initial Org commits.

I think it would make sense to change it.
However, if we change special treatment of the first/last characters in
org-time-stamp-custom-formats, it will also make sense to change
org-time-stamp-formats constant.

For backwards compatibility, we will need to keep special treatment to
strip brackets around the formats, if present.

I propose to do the following:
1. org-time-stamp-formats and org-time-stamp-custom-formats will be
   treated as is, unless they contain "<" and ">" and the first and the
   last char.
2. If the formats do contain <...>, strip the "<" and ">".
3. Document (2) in the docstrings.

Any objections?

-- 
Ihor Radchenko // yantar92,
Org mode contributor,
Learn more about Org mode at <https://orgmode.org/>.
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