Ask HN: How to do create reminders in your app?

I am seeing the need for a flexible reminder system in our app to remind people about various things they should do.

It's not an app that people use daily, so it's important to remind them about pending tasks and upcoming deadlines. I can find several services to deliver the message, but nothing that would let me generate the message without writing a lot of code.

Have you built something like this or aware of an API that can do this?