Adding minutes top time helper by use of buttom

Configuration Frontend
,
1

Hi,
I have a daily automation where time needs to be changed frequently.
Therefore I use a card + grid with the time in the middle and “+” and “-” left and right.
I would like “+” to add 15 minutes to the time and “-” to substract 15 minutes of the time.
Is this possible to do this inside the card? If yes, can someone help me on this?

type: grid
cards:
  - type: button
    icon: mdi:plus-box
    name: ' '
    tap_action:
      action: call-service
      service: input_datetime.set_datetime
      service_data:
        # add 15 minutes to input_datetime.action_time
      target:
        entity_id:
          - input_datetime.action_time
  - type: entity
    entity: input_datetime.action_time
    name: Action
  - type: button
    icon: mdi:minus-box
    name: ' '
    tap_action:
      action: call-service
      service: input_datetime.set_datetime
      service_data:
        # substract 15 minutes from input_datetime.action_time
      target:
        entity_id:
          - input_datetime.action_time

Thanks a lot!
Christian