Tagging in a chat app seems like it's trying to make a chat app be something it's not. Unless I'm wildly unfamiliar with how many people use chatting. (totally possible!)
That said, the timeline jump is the idea in here that made me say, "ok that is slick" I feel like I sometimes want to go back and find a photo someone sent me several days ago and I, well, don't even bother because I won't even remember exactly WHEN it was sent.