ISY Newbie,
Since I am a newbie myself, I will talk "Newbie" to you as that is the only language I know...
You are absolutely right - the From/To phrase makes you think that it will check continuously. I thought the same thing myself. However, it doesn't (thank you MrBill for explaining it to me). The program evaluates only at those 2 times (sunset and next day's sunrise). When lilyoyo1 said the ISY is event driven, he means that something has to happen (a certain time, light-on, light-off or, really, any condition that the ISY knows about) has to occur which then triggers the ISY to go through its programs to see if any program should be evaluated because something (an event) happened.
Your original question was about the ISY checking continuously throughout the night and turning a light back on "at some point" if it's turned off. Since the ISY won't check continuously but, with a tweak, can evaluate its programs when the light is turned off (this is considered an event), we need to incorporate that info into the program. Asbril's first post has the appropriate program. He just used "Wait 2 minutes" because you said "at some point". However, you can use any interval you want with a minimum of 1 second.
TrojanHorse - post #2 is completely accurate; however, it's written above newbie level. I understand what you said there, but I don't think it would have sunk in during my first few weeks of programming.
I hope this is helpful,
Ross