The activity schedule is defined inside the NUI package in the ActivitySectionV2 module, as an element in a diff object named Schedule. Bound methods (everything after bindTo, for example getScheduleItemTitle) are defined inside the UIv2 package in the ActivitySectionGridRowViewModel module.
The internal logic of the schedule items is defined in the schedule-item.js file (Terrasoft.controls.ScheduleItem class).
Check those files for this marker.
How to override schedule-item.js to implement the above feature ?
You can use development in the file system to modify this file as it is a part of system resources.