Your question is Scheduler API Design. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
Design a Scheduler API (given a timer and timer callback API method).
Clarify the API semantics, scale assumptions, and reliability guarantees you would provide for creating, updating, cancelling, and firing timers. Focus on how you would handle ordering, retries, duplicate callbacks, and failures in a distributed system.