Well if you're allowed to add a timer that's in the past, you should be able to delete it. *Adding* timers in the past is more interesting. I guess we could reject them in the timer store, but then chronos would need to reject them back to the client (but what if it's a replication), or for a repeating timer re-add them with the next pop time. 

It's not immediately obvious to me what the right approach is. I suspect if we raised this as a separate theoretical issue we wouldn't do anything about it unless we had some evidence of real negative impact. I don't think it's worth making any changes right now.