Timestamp modification for Sensor

Hi There,

I have a sensor, which will report the estimated time when charging the car will be finished.
This sensor provides an Timestamp (UTC) - so while using the Timestamp as it is, I am getting not the correct time displayed in my dashboard (since I am not in a UTC time Zone).

Therefore, I want to create a template sensor to format the Timestamp:

  friendly_name: "Vorraussichtliches Ladeende"
  value_template: >-
  {{ as_timestamp(states('sensor.openwb_cp1_voraus_ladeende'), now() | as_timestamp()) | timestamp_custom('%d.%m.%Y %H:%M', true) }}

is my current setup - which will provide the following Output:
“Vorraussichtliches Ladeende”: 17.06.2022 08:20

Currently, the sensor is showing “unknown” - when no charging takes place - therefore, I am running into an issue when trying to convert the timestamp in that situation.
Is there a way, how I can set the sensors default value back to “unknown” instead of the _now() | as_timestamp?