No, I don't believe the sabbath is a sun up/ sun down thing. More of a midnight to midnight thing.
I must say, not all mormons feel trick or treating on Sunday is bad, and it is not fair to lump them all in a group and accuse them of being unkind. In fact, many I know are willing, but they have gotten the vibe that the neighborhood is all doing it on Saturday, so they are just going along with it. Really, there isn't anything wrong or "dark" about visiting neighbors and getting candy. It's all in fun.
I only shared that this happens in other communities for religious AND non religious communities across the US to offer some perspective. Blame who you want for the confusion here in UT, but this isn't unique to our area. I really wish Daybreak would designate a day so we can all enjoy and make the most of it, as well as ensure the safety of those that are out as well. In towns I have been in that changed the date ( once for a Big 10 football game in a small college town! How dare they! haha)they simply designated the date and time for trick or treating and it wasn't a big deal. No one's feeling were hurt and everyone was out at the same time. Then, on actual Halloween, people had fun dinners or parties to celebrate. It worked well.
Hmmm, maybe we will go out trick or treating both nights to ensure we get everyone's house . .. haloween candy . . . yum!