Hi Marnix,
I thought I'd check your test case in the Github test project. Even though I wasn't able to get your 'object Object' in my Chrome, I did manage to stumble on to some issues with the date/time format setup. I'm working on a fix right now, but until that's released try this:
Make sure the CustomTimeFormat isn't set if you use View specific time formats, and the other way around. This should hopefully cover you for now!
Keep in mind that the calendar uses a JavaScript library for date formatting. So the date format pattern and options aren't the same as in microflow.
If I recall correctly the calendar widets uses lower case for hours, in an older version of the calendar widget I was able to use the custom expression hh:mm
The message: "object Object" is probably caused by an exception. If you open the developer tools (chrome), in the console tab there should be a bit more info about the error?