How to execute code after start-process completion?

[Pankaj Jangid <pankaj@codeisgreat.org>]

I am working on tiny customization of my Gnus. And using the function
`start-process` to call `notmuch new`.

#+BEGIN_SRC emacs-lisp
(add-hook 'gnus-after-getting-new-news-hook
	  (lambda ()
	    (start-process "notmuch" "*Notmuch*" "notmuch" "new")
	    ;; (print (current-time-string) (get-buffer "*Notmuch*"))
	    ))
#+END_SRC

After completion of the process (`notmuch new`), I want to execute some
code. As you can see above, I want to put the timestamp in the *Notmuch*
buffer. I know that `start-process` returns the process object. But I
couldn't find how to use it. Please share any pointers/code-snippet on
how to use that object accomplish what I want to do? Or may be there is
a different way to do the task. Please advise.

-- 
Pankaj Jangid