Yes, 22nd isn't an exact date, for two reasons: 1) it depends on us getting a version of hivemind ready for apps testing in the next few days and 2) witness don't have to upgrade right away, in which case it could take longer. So what it really means is that Sept 22nd is earliest possible date that we will hardfork, but not necessarily the day it will happen. Testing and witness upgrade cycle will set the actual date.
Regarding notifications, it's a low-level coding thing related to hivemind itself. Front ends like hive.blog have a feature called "notifications" which allows you to know when someone has, for example, "mentioned" you, rehived your post, or upvoted one of your posts. There can also be notifications as a result of actions performed in a community. These latter types of notifications need special handling in hivemind, compared to the other ones.