Ask HN: How do Google calendar notifications work?

My work uses Google Calendar. I'm amazed that I always get notifications for meetings on time and in multiple mediums (i.e., email, push notification, etc).

How is a notification system like this built? I've been poking around for time-based event systems, but I don't think that's the right thing to look at.

Finally, how can you scale this? I'm just imagining a ton of distributed cron-y things, but I know that's naive.