It appears that’s the default date format for month views in the calendar for the eventTimeFormat : https://fullcalendar.io/docs/v4/eventTimeFormat
I see @psweb had the same issue some time back: Event Calendar 24 hour return but it never got answered.
I can’t find such an option available in the UI, so maybe @patrick can check if we can update the calendar and add such an option to it.