iPhone Displays Invalid timestamp

I made a timestamp sensor that uses as its main value the last changed state of another sensor. In a normal web browser this is displayed correctly but on my iPhone whether it is in the app or through the browser it just displays “Invalid timestamp”.

- platform: template
      friendly_name: "Door Bell"
      device_class: timestamp
      icon_template: "mdi:bell-outline"
      value_template: "{{ states.binary_sensor.door_bell_contact.last_changed }}"