Define ttf before it's used

[blalor]

No description provided.

[Dieterbe]

this code change would hide problems where your first outage event is not a start event.
instead of silently allowing this case, we should actually even react stronger and report to the user that if they have an outage event, the first one must be a start event. (a side-effect of that is also that ttf is set in the first run)