Add date to mu4e email when forwarding or replying

Question

I use Emacs and mu4e for email. If I forward an email, the body defaults to this:

From: me <>
To: 
Subject: Fwd: ...
--text follows this line--


Sender <> writes:
...

I would like mu4e to add the date of the email forwarded, such as:

On 1 October 2019, at 18:07, sender <> wrote:
...

I looked at the source code of mu4e-forward, mu4e-compose, and mu4e~proc-compose without seeing where the text is added. I think it's in:

  (mu4e~proc-send-command
    "cmd:compose type:%s docid:%d extract-encrypted:%s use-agent:true"
    (symbol-name type) docid (if decrypt "true" "false")))

but I don't know how to hack "cmd:compose ..."

Has anyone tweaked the default message when forwarding or replying?

Answer 1

mu4e uses message.el to edit the messages. The documentation for message-citation-line-function in that package shows:

Function called to insert the "Whomever writes:" line.

Predefined functions include ‘message-insert-citation-line’ and ‘message-insert-formatted-citation-line’ (see the variable ‘message-citation-line-format’).

The definition of message-insert-citation-line has:

(insert (mail-header-from message-reply-headers) " writes:")

But this one has no formatting to customize with date and time. The definition of message-citation-line-format shows a value of:

On %a, %b %d %Y, %N wrote:

So you'll have to set both message-citation-line-function and message-citation-line-format:

(setq message-citation-line-function 'message-insert-formatted-citation-line)
(setq message-citation-line-format "On %Y-%m-%d at %R %Z, %f wrote:\n")

The FAQs mention this customization to avoid a bug in Outlook:

(setq  message-citation-line-format "On %Y-%m-%d at %R %Z, %f wrote...")`