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.

Daniel