timer:
  laundry:
    #duration: "03:20:00"

sensor:
  - platform: template
    sensors:
      laundry_remaining:
        friendly_name: "Laundry Remaining"
        value_template: >
          {% set f = state_attr('timer.laundry', 'finishes_at') %}
          {{ '00:00:00' if f == None else 
            (as_datetime(f) - now()).total_seconds() | timestamp_custom('%H:%M:%S', false) }}
template:
  - trigger:
      - platform: state
        entity_id:
          - timer.laundry
        attribute: remaining
    sensor:
      # Keep track how many days have past since a date
      - name: laundry_remaining
        state: >
          {% set f = state_attr('timer.laundry', 'finishes_at') %}
          {% set res = '00:00:00' %}
          {% if f != None %}
          {{ (as_datetime(f) - now()).total_seconds() | timestamp_custom('%H:%M:%S', false) }}
          {% else %}
          {{ res }}
          {% endif %}