Only update sensor when travel time changes

Configuration
1

Hi there, I have the following challenge. I have a sensor that is named sensor.traveltime, this contains my travel time to home in minutes. Becasue I want to show my arrival time on a dashboard, I created a custom sensor using this code:

  - platform: template
    sensors:
      arrival_time:
        value_template: '{{ (utcnow().strftime("%s") | int + state_attr("sensor.traveltime","duration") | int*60) | timestamp_custom("%H:%M:%S") }}'
        friendly_name: 'Arrival Timel'
        icon_template: "mdi:clock-time-four"

But this sensor doesn’t work that well, because the travel time only gets updated once every 5 minutes. But the arrival time sensor updates every time that utcnow changes. So the arrival time becomes 1 minute later every minute, untill travel time sensor is updated, and then arrival time becomes 5 minutes earlier again.

So bottom line, I only want the arrival time sensor to update, when the travel time changes. Is that possible? Do I need to use an automation maybe?

2

Yes it is. Use a triggered template sensor.

https://www.home-assistant.io/integrations/template/#trigger-based-template-sensors

3

Can you help me on my way a little bit? I gues my trigger should look something like this:

template:
  - trigger:
      - platform: numeric_state
        entity_id: sensor.traveltime

But HA tells me I need at least one below or above:
Invalid config for [template]: must contain at least one of below, above
But I want it to trigger on any change, doesn’t matter what.

For the sensor, would this work?

    sensor:
      - name: "Test Arrival TIme"
        state: '{{ (utcnow().strftime("%s") | int + state_attr("sensor.traveltime","duration") | int*60) | timestamp_custom("%H:%M:%S") }}'
4 
template:
  - trigger:
      - platform: state
        entity_id: sensor.traveltime