I have used the UN Job list website and I would like to thank you for initiate that.
Timeline of mine:
Organization: Secretariat @ New York
Post Level: G-4 (probably much different from P level jobs)
Application: Late May 2017
First Call: Mid July 2017
Test: Mid August 2017
Interview: Late August 2017
Administrative Process: Early September 2017
Report to Work: Early October 2017.
But I have another colleague said she knew someone who applied to this post and got the interview in early July. I feel it is highly possible that that person is the first choice and for some reason, it did not work out and they moved to me (since I got the phone call in mid-July). Just a guess.