Steps to reproduce:

1. Configure host, item, trigger.
2. Force the trigger to go PROBLEM state, remain in PROBLEM state for some time (10 seconds is enough) and then return it to OK state.
3. Go to Monitoring -> Trigger page, filter for any trigger and show events for 7 days, expand the event information as shown in the attachments.

Result:
In 4.0.0alpha3, duration will be equal to age of a problem.
In 3.4.7, the duration will be either equal to age of a problem or will contain different incorrect duration.
In 3.4.8, the duration will be equal to age of a problem with negative value.

Expected:
Correct duration is displayed, same as in event detail view (as seen in attachments).