Design a URL Callback Scheduler

I need to design a URL Callback Scheduler system for an application with potentially millions of jobs per days; the scheduler will need to do the following,

Provide an API for clients to register a URL Callback to be called at a specific date and time, the callback time is between 1 minute to 1 year; in other words, the client can register a callback to be fired in 1 minute in the future, or a year in future.

My questions are,

1- Is there a design pattern that I can utilize?
2- Are you aware of an open source application that does this? 

I've been searching for days to get a clue on how to start but haven't found anything useful, you're help is greatly appreciated.