I don't feel appropiate changing global crawling setting for the whole site just for issues raised by a single plugin. In Webmaster Tools setting the URL parameters days, start, startoffset, events
to No URLs
might be enough. Here's the official guide: https://support.google.com/webmasters/answer/1235687
Still, I'm convinced there should be a better and easier solution to this, preferably coded in the plugin by default.