The alert time column indicates how many seconds have passed since first localized alert from this investigated alert group to this particular alert.
E. g. if we have 3 alerts with alert_id AAL with alert times: 0, 1311, 1319 this means that:
1311 seconds have passed from first alert to second alert
1319 seconds have passed from first alert to third alert
If you have any other questions, feel free to ask.