Take a look at this for a custom business calendar implementation that supports various implementations https://github.com/StephenOTT/forms-manager/tree/master/src/main/kotlin/formsmanager/businesscalendar
The trouble with current Camunda impl is the timer calculation code is mostly hard coded to specific use cases and injection of new logic is painful: so mostly you need to completely replace some of the internal classes. The above code should give you a idea of some of the utility and where you need to make edits