Based on Stephen Kitt's helpful answer I added this to my
set attribution="On %[!%F at %T], %n (%a) wrote:"
Which produces a line like this in the reply:
On 2019-11-20 at 19:31:16, The Sender (firstname.lastname@example.org) wrote:
I first tried to set
date_format using Mutt's hooks (
folder-hook) in order to use
%d in both
attribution and the
index_format but still have different time formats for index and
attribution. This worked until returning to the index from replying to a message: The index would now show the same time format as the attribution line in the message reply.
Since there doesn't seem to be a
post-reply-hook or a hook that's executed when showing the index, I used the square brackets to format the date in the
attribution setting, something I learned how to do here.