Hi Chui,
How about a mechanical device?
A clock could be adapted to operate a light sensitive sensor.
It is possible to make an oscillator which will run at such very
low frequencies, but they are intrinsically inacurate, and not
easy to make. Circuits having such a low natural resonance require
very ponderous component values.
If accuracy is necessary then Nigel's suggestion of outputting the
values over a half hour period, using small increments, would be
much more practical, and also as accurate as you care to make it.
If this thing has to be made in a hurry, i would use an old clock
with the minute hand changed for a disc of card cut into a shape
with a light on one side and a sensor on the other side.
Cheap and cheerfull, and as accurate as you can trim the card.
Best of luck with it, John