According to the state-owned MONTSAME news agency, Sep 24 00:00 is correct:

http://en.montsame.mn/video/society/winter-dst-start-weekends

"Ulaanbaatar /MONTSAME/ At the night from September 23 to September 24, Mongolia will turn its clocks one hour back to practice the daylight saving time (DST) for the coming winter, in accordance with the government decision." - B.Amarsaikhan - 09/23/2016 - 10:58

On Fri, Sep 23, 2016 at 3:54 PM, Paul Eggert <eggert@cs.ucla.edu> wrote:

On 09/23/2016 11:23 AM, World Time Zone wrote:

There are conflicting reports that Mongolia ends DST on Sep 24, Sep 26 or October 1, 2016?

Ouch. tzdata says Sept. 24 at 00:00, which would have taken place about four hours ago. Is there anyone one the ground who can confirm the correct transition time?

I'll CC: our Mongolian email correspondents from last year to see whether they can help. If tzdata is wrong there are a lot of cell phones in Mongolia that now disagree with official time.