`cron` or `at` for the Cloud?

For a live-chat social system, we wanted a feature where users should be able to create events, like "party at 3pm on Sunday". And we'd like their friends to be notified of those events, shortly before they occur.

This isn't super hard to build but I was surprised that there wasn't a generic service for scheduling events to happen in the far future. There's no standard way to (for instance) trigger an AWS Lambda function to run at a particular time in the future, with a particular set of arguments, that might send a notification, or an email, or call another API.

Is there anything like this? Would you find it useful if it was a SaaS or something you could add to your hosted cloud services?