Time calculation hour addition

Configuration
4 
  - service: input_datetime.set_datetime
    data:
      timestamp: >
        {{ (today_at('12:00:00') + timedelta(hours = states('input_number.piscine_nombre_fonctionnement_heure') | float(0))).timestamp() }}
    target:
      entity_id: input_datetime.piscine_debut_periode

If that fails to work (potentially because the template reports a timestamp value that includes the current date) then use this one:

  - service: input_datetime.set_datetime
    data:
      time: >
        {{ (today_at('12:00:00') + timedelta(hours = states('input_number.piscine_nombre_fonctionnement_heure') | float(0))).strftime('%H:%M:%S') }}
    target:
      entity_id: input_datetime.piscine_debut_periode