What is the other number in the alarm notification?
The time on the right-side of the notification is - just like all other notifications - the time when the notification appears.

The alarm notification is a new feature from Android stock Clock app on KitKat.


  Google's latest Clock app ... now notifies you two hours ahead of when your alarm is set to go off ...


The clock app will display the first notification 2 hours before, then the second one 30 minutes before the alarm goes off.

By looking how Android works on the notification, when the notification is displayed using same ID and it's not dismissed yet, the notification will be updated instead and the "time when notification appears" will also be updated (overwritten) to the new (current) time.

In your case, when you set the alarm for 55 minutes, the notification appeared since it's already less than 2 hours. Assuming you didn't dismiss the notification, when the alarm was less than 30 minutes, the app updated the notification. Since there was no info changed (alarm's time &amp; name are still the same), you wouldn't even notice that the notification was updated, except the "time when the notification appears".