What sensor are you trying to create? The calendar state is on if the event is currently started and off when there are no events occurring. The calendar also has these state attributes which can be accessed without having to be created.
I use the start_time state attribute to create a sensor of the seconds till the next event start time.
- platform: template
sensors:
timetoleavesec:
entity_id: sensor.time
value_template: >-
{{(as_timestamp(states.calendar.calendar.attributes.start_time) - as_timestamp(now())) }}