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).