Oh, thanks. My fault! I just try to use the state object. this works perfectly.
Strangely, the variables behave differently in the blueprint than in an automation.
In the automation last_triggered is updated after the call and in the blueprint before. So in the blueprint last_triggered is always the time of the current call and not the last one.
//EDIT: so I found the difference
current trigger: states[this.entity_id].attributes.last_triggered
last trigger: this.attributes.last_triggered