Everyone has their own debugging techniques but the script debugger in FileMaker Advanced is really essential for following along with scripts as they're executing: you can tell if your script is on the layout you think it is, and importantly, what variables are available to you at any point in a script (via the data viewer).
Even though DayBack's layout is rendered in a webviewer, when the calendar interacts with your data it's still running FileMaker scripts, so you can still use the debugger.