Scheduling a notebook task allows users to specify the period the task will run during the day.
It does not indicate that this will run in UTC and this is not specified in the documentation.
https://doc.arcgis.com/en/arcgis-online/create-maps/prepare-a-notebook-for-automated-execution.htm
This is causing difficulty as users are scheduling tasks to run every 15 minutes during the overnight period in non-UTC timezones and it's not immediately apparent that this behaviour will occur.
It also means that in Australia we can't run things during business hours only as it's 'overnight' in UTC.