Template sensor with off delay is unknown after restart - remedy?

I have a template sensor that has an off delay of 900 s.

  - binary_sensor:
    - name: "test"
        seconds: 900

After every restart of Home Assistant it stays unknown for 900 s. Is there a way to give it a default value or keeping the value so I don’t have an unknown sensor? Ideally it would save the state from before the restart which is usually right.

I know this behavior is correct, but I still want to fix that.