Using
nndiary manually (I mean, writing the headers by
hand and so on) would be rather boring. Fortunately, there is a
library called gnus-diary written on top of
nndiary, that does many useful things for you.
In order to use it, add the following line to your ~/.gnus.el file:
(require 'gnus-diary)
Also, you shouldn't use any
gnus-user-format-function-[d|D] (see Summary Buffer
Lines). gnus-diary provides both of these (sorry
if you used them before).