You could do this:
alias: Robotics club
description: ""
trigger:
- platform: time
at: '15:00:00'
condition:
- condition: state
entity_id: calendar.robotics
state: 'on'
action:
- service: input_boolean.turn_on
target:
entity_id: input_boolean.robotics_reminder
mode: single
Alternatively, you could ditch the condition and replace the trigger with a calendar trigger:
trigger:
- platform: calendar
event: start
entity_id: calendar.robotics
offset: +15:00:00
The second option feels weird though, but that might just be because I’ve never used a calendar trigger before.