Many BTTF fans know that Marty, Doc and Jennifer arrive in Hill Valley on October 21st 2015, and were eagerly awaiting that date.
As outlined in this questionthis question, they arrive at 4:29PM. However, the Earth is divided into timezones. So, in order to figure out what time 4:29PM on October 21 2015 is around the world in other timezones, we need to know where Hill Valley is, more specifically, what timezone Hill Valley is in.
As I cannot recall a reference to where Hill Valley is, my question is: where is Hill Valley and what timezone does this put it in?