I use LightTrac for the iPad. Not only does it tell you when but also the angle from which the sun will be coming at sunrise, sunset and right now, as the sun position varys seasonally. Besides, it’s cool to be able to drag the slider around and see how the direction of the sun rays will vary.

So say I want to photograph the tower in the middle of the plaza in this photo. I can see there will be a cast shadow right in the way at 5:30PM just because I can drag the slider over close to sunset. (Believe me, picking the optimal time for this particular shot is a PITA.)

Point is, this app gives you not only the time but also the direction of the light.

Hope it’s useful and even if you’re not using an iPad, you can think about directionality of the light.