I believe that would be America / Phoenix. It's a little awkward how the US isn't partitioned into a seperate group, the names are based on the tz database. I might have a look at how invasive it would be to change the grouping a bit, but I don't want to mess with things too much, since I'm a little worried that I'll be ignorant to some detail and end up wiping out someones time zone by accident/introducing bugs down the road
yeah it's based on regions that have always shared the same exact time since at least 1970, and the name is just the most populous place in that region (or so I read). so it's not just for people in phoenix, that's just what they decided to name that time zone area after.