Ah yikes that’s a missed opportunity on my part. It’s https://re.markwhen.com and it’s slightly broken at the moment. Basically it turns markwhen into a blog, each entry is an event. Only entries that are before the current time show up.

You just add ‘re:’ in the header of a shared markwhen and (when I fix it) it shows up as re.markwhen.com/[your username]