alarm.yaml

input_boolean:
  alarm:

input_datetime:
  alarm:
    has_date: false
    has_time: true

automation:
 - alias: set alarm
   trigger:
     platform: state
     entity_id: input_datetime.alarm
   action:
    - service: input_boolean.turn_on
      entity_id: input_boolean.alarm

 - alias: run alarm
   trigger:
    platform: template
    value_template: "{{ states('sensor.time') == states('input_datetime.alarm')[:5] }}"
   condition:
     - condition: state
       entity_id: input_boolean.alarm
       state: 'on'
   action:
    - service: light.turn_on
      data:
        entity_id: light.0x086bd7fffe03d84e_light
        brightness: 255
        transition: 600
    - service: input_boolean.turn_off
      entity_id: input_boolean.alarm

ui.yaml

- title: alarm
  path: alarm
  icon: 'mdi:clock-time-seven-outline'
  badges: []
  cards:
    - type: entities
      show_header_toggle: false
      entities:
        - entity: sensor.time
        - entity: input_boolean.alarm
        - entity: input_datetime.alarm