A
Can i let the sunset start at a custom hour?
No, only God can do so. However, if you have severe problems with the time the sun goes down, you might consider moving to another part of the world.
I'd like to know how to let flux start at 8 PM.
Disable 'start at login' and use a utility like Keyboard Maestro to start f.lux each day automatically at 8 PM.
I'd like to know how to let f.lux start the change from Daytime mode to Sunset mode always at 8 PM.
That is not possible, but why would you want to do so?
f.lux serves to adjust the color temperature (that is the relative amount of red and blue) of your screen to the lighting of your working environment. That lightning typically changes considerably around sunrise and sunset, so it makes sense to change color temperatures around those times, rather than at a fixed time.
And is there a way to set a custom 'line' for the light changes
Not that I know.
Again, why do you want to do that? What is the problem you want to solve?
I am asking this because I noted that you asked this question several times on the forum and didn't get an answer.
Apparently, you have a problem you want to solve. However, your post doesn't describe that problem. It only tells us what you think would be the solution (namely, customizing the start time of the sunset mode) and asks how to implement it.
Unfortunately, your alleged solution doesn't make sense, because the whole point of the sunset mode is to adjust the screen's color temperature to the difference between a room lightened by the sun and one lightened by artificial light.
Given that your solution doesn't make sense and you didn't describe the problem you're struggling with, it is difficult to give a useful answer to your question.
You might get more and better answers if you start a new topic in which explain your problem, rather than an assumed solution.
