I set up an alarm (using Google Now) for 55 minutes, so the end time was around 20:13 (and therefore the start time was 19:18
When I looked at the alarm notification, there's another little number in the right hand side which said 19:43 (so 30 minutes before the time the alarm was set for)
It would make sense if it was the start time (19:18) but by itself it makes no sense.
Edit: I just tried it again and the same thing happened. I set the alarm for 55 minutes at 20:36, and it says 20:36 on the right hand side of the notification, then when there's 30 minutes left, it changed to 21:01 (which is 30 minutes before the end time)