This is actually expected behavior - in the USM alarm table view, the 'Time' column reflects the most recent activity on the alarm, such as being assigned or an increase in severity, or another instance of the same alarm, etc.
In order to view the original alarm time of arrival, in USM you can take the following steps:
1. In the alarm view go to Actions->Edit Columns...
2. In the popup menu choose to add the column 'Started'
This column will display the original time of the arrival of the alarm. Otherwise the 'time' column shows the most recent activity on that alarm.