I suppose it would need some way of figuring out how long it's been disconnected, or it would need to keep time without external power, so it can set a new time. Or alternatively, it could just show the last time that was shown. Although I think it's odd, I suppose it makes some sense to store presets and other mostly static information in capacitor-backed storage, while something like time will only be kept when power is supplied.
Eh, I've mostly answered my own question. Or maybe I just don't care anymore?
Never mind. Move along.
Flash memory for everyone!