I've seen such devices (and in fact have one, by TimeGuard)
but they all seem to be far too clever,
and have various modes switching on and off at random,
but amazingly do not have a mode which simply switches on
when it is dark, and off when it is light.

I had such a device, which came free with my house insurance one year. So
they do exist. It was very simple with no controls at all. Even worked with
low energy bulbs. (This should be an essential feature of such a device,
really).

Christian.