I had this issue too, on my VZW iPhone. Had nothing to do with any alarms that were set. I left work at 11:30pm, my phone had the correct time. I got home at 12:14am, and I noticed that my phone was now reading 11:14pm. As I had been driving in between those two times, I didn't notice when it made the switch, but I assume it was at midnight.
I went into settings and I noticed that my timezone said "Chicago", even though I live in NY. I deleted Chicago and set it to New York. The time switched to the correct time, and then a moment later fell back an hour again! Then I turned off "Set Time Automatically" and set it manually. That set the time to the correct time. I was curious to see if it would ever fix itself, so I set it back to Automatically, and then went to bed. I woke up in the middle of the night and saw that it was still an hour behind, even though DST took effect at 2am. However, when I woke up this morning at around 11am, my phone was correct (as in, it matched my cable box which I assume updated automatically).