While the creation of a ticket is straight forward, updating an existing ticket is a bit trickier. In terms of triggers, a ticket is only updated if you press the update button on the lower right of a ticket. Adding tags to a ticket or switching articles visibility is no ticket update.
Also keep in mind that we’re always only working on the last article. This means you can’t trigger for past articles. Triggers always handle the current ticket attributes and the article (if applicable) that cause the trigger to fire.
If your use case doesn’t fit in above possibilities, you might want to have a look at Zammad’s Scheduler.