@aaronpk Congratulations on moving a big piece of IndieWeb infrastructure forward a generation or so :-)
I’m curious about your decision to omit timezones from multi-day events; won’t that cause those events to show up incorrectly for people on the other side of the International Date Line?
Yes and no.
If you want to attend an event in Portland on June 29th, and you live in Japan, and you add that event to your calendar with no timezone, your calendar will record it as "June 29th". That means no matter what timezone you switch your phone to, it will always show up as the right date.
It only fails if your phone adds a fixed timezone to it such as the timezone of your current location. In that case, it will show up at a different time once you change timezones.
Also this is not unique to the date line, it happens with any timezone change. The important thing is that if there is no timezone, calendar apps need to treat it as a "floating" or "local" time and always display the stated time of the event completely ignoring timezones.
The case where it doesn't work is if you want to remotely attend an event in a different timezone. That's why for short one-day events that are likely to have remote participants, I recommend setting a timezone on the event. But for multi-day events, you're going to have to look up the starting time on the actual schedule anyway, so you'll figure out the timezone conversion at that point instead.