I don't want a list of events with created_at after my 'since' filter. What I really want is all the events that the relay has become aware of since I last got events off of that relay. But we can only ask for one or the other (and I'm not clear which one relays are using).

Event 'created_at' stamps are created by the author at time A. Relays may become aware of these events quite a bit later on, at time C > A. If I last fetched events from the relay at time B < C, and I come back and ask for events since time B, will I get the event created at time A < B? My assumption is that I will not, and the only way to pick these up is to just have a huge overlap time in my request and hope I don't miss much.

If I'm correct and 'since' refers to 'created_at', I would offer to write a NIP that extends filters with another field. Relays would have to record when they became aware of the event (seems pretty easy, a timestamp default now field in a database).