Ask HN: How does one design a system to deliver scheduled notifications?

Every week at work I get a bunch of reminders which I schedule in apps like Slack, Google Calendar, etc. These notifications arrive always within the minute that I scheduled and it always impresses me! Delightful!

I want to learn about how a this type of system is designed. I've searched a lot, but haven't yielded anything about systems that handle scheduled notifications.

Some wonderings: - What is the name given to these types of systems (if any)? - How does one design a system like this? - Are there any open source projects I can learn from?