Hill Valley is in California, Pacific time with daylight savings (UTC-0700),
so the moment of truth is 11:29pm UTC.
The following 1885 railroad map, appearing in Back to the Future III (and listed here as a prop), shows that Hill Valley is located in northern California in the Sierra Nevada mountains:
(As @RoyalCanadianBandit points out, the DeLorean also has Californian license plates; however, on its own this is not certain evidence, as the car might have had plates from a different state.)
California is in the Pacific time zone. And daylight saving time in the USA ends on 1 November 2015, so the time was still 1 hour ahead on 21 October. Thus 4:29pm in Hill Valley is 11:29pm UTC.
This is confirmed (non-canonically) by the website Is today Back to the Future Day?, from which a screenshot taken at 8:07pm UTC on The Day (21 October 2015) looks like this:
For the pedantspedants: under the rules in effect in 1989 (after the 1986 enactment of PL 99-359 to amend the Uniform Time Act, but before the Energy Policy Act of 2005 - see this Wikipedia article on daylight savings), US Daylight Saving Time would have ended on the last Sunday in October instead of the first Sunday in November. In 2015 this is 25 October, which is still after 21 October, so Daylight Saving Time would still have been in effect and the answer remains the same.
