but was not able to post it, since it contained a link. So I have to start all over
Any time you're writing a long/complicated post to a website, you should copy and paste it to a text editor window. Maybe even save it to disk if there's a thunderstorm going on, or your roommate is testing the high-power industrial laser she bought at the bankruptcy auction. You never know when a blog software bug, a power failure, or the electromagnetic field from a passing alien spaceship will cause a problem with posting.
I think you've already figured out that variations like clouds can cause problems with trying to determine sunrise and sunset from light level. But it sounds like the ambient light level is what you really want to measure, anyway, since you're controlling artificial lights: if the clouds are especially thick, it might get dark enough that you'd want to turn on some of the lights at 0930.
You're probably going to need to put your light sensor in its own little enclosure, so it can be pointed where it will truly measure daylight: you don't want the clock turning off the porch light because you sat down at your desk and turned on the reading lamp next to the clock :) Or turning it on because you brought a box to your desk and set it down so it blocked the sensor. I would put an opamp, wired as a unity-gain buffer, in the enclosure with the sensor: it will give you a nice, strong low-impedance signal to feed that ADC, and reduce the risk of the sensor cable turning into an antenna.