We can all agree that the time in NY is 3 hrs later than the time in Los Angeles, but I was arguing to a friend that the way to describe this is that NY is “behind”. …err …I’m kind of forgetting my justification for this now. His explanation was that they arrive at any time earlier than LA, so they arrive ahead.
I’ve always heard it described as “NY is 3 hours ahead of L.A.”
But then again, I’m from the East Coast so maybe there’s some bias? 
Here in New York, I and everyone I know (a sizable, but yet not necessarily representative group) always say we are 3 hours ahead of, say LA, and 5[or whatever] hours behind, say, London.
I’ve never heard NY described as being behind LA and I deal with LA companies all the time (I’m on the East Coast). It is behind London and anyplace farther east than NY, though. Maybe you are confusing the terminology somehow?
Definitely “ahead of.” Just like I’m 5 hours ahead of EST.