The "Geeks" will love this...when does the sun go down?

[waterbear]

If you are an orthodox jew it is the time it goes down in Israel! (actually some truth in this!)

[Henry3]

And the Moslem definition:
"dark" is traditionally defined as "when you cannot distinguish between a black thread and a white thread".

[sailork]

I bought a second set of trippers for my intermatic timer. I have it run several hours in the morning after sunrise, and then I have it run two hours in the late evening. That way the pool doesn't go as long between circulations. Right now it kicks on just before 10 so I can run out there and dump some bleach in before the Daily Show. The nice thing is that the timer will turn it off for me so I don't accidentally leave it run all night.

As far as your question is concerned as the sun begins to set there is so little light (compared to noon) reaching your pool that it would probably make no difference at all if you added the chlorine then. It has to do with the amount of atmosphere the light has to travel through and the incident angle on your pool. I only add mine so late because thats when the trippers are set.