This timer is quite simple (also sold under the Woods name).

1) Plug in what you are powering
2) plug in the timer
3) display should blink with top and bottom red LED bars and power should be on
4) press the only button it has once, to set it for on at dusk-off at dawn (red bars solid, not blinking)
5) more presses will let you set how many hours (LED digit shows now) to stay on AFTER dusk

NOTE! After setting the duration on time, the timer stay on for at least two minutes (even in broad daylight). This is to make sure your done setting it. Set it and leave it for at least 5 minutes and if there is enough light, it should go off.

ALSO! Don't set the light it is powering close to the light sensor (photo-cell in small round clear window above the red LED display). This will cause the timer to stay on as the light constantly activates the light sensor

A manual can be found here -> http://waterheatertimer.org/pdf/Woods-49103-N1502-manual.pdf (Thanks Geno_3245)