If your event "system time may now be considered stable" shall be about NTP then it makes little sense since NTP clients tend to slowly adjust the time instead of making it jump. That means that the time is adjusted fully only after quite a bit of time. Applications should not wait for that. They should just assume that time is correct, right from the beginning. And that is a safe assumption for all machines built in the last 25y. And again, I don't see what systemd has to do with eventing like that anyway. I see no need to multiplex events through an eventing system. Get your events from the respective subsystems directly. Don't add an indirection layer here.
People should handle "in-lifetime" events (as you call them) with the native notification logic available. no need to involve systemd, or dbus or anything.