Hey 123, you don't have to have any certain time to switch to 12-12, day or night does not matter .
when they are big enough, switch the light to 12-12. ,
Forget about the dark period. So if you go in there and look at them, and they are tall enough, switch the light, does not matter if it's day or night. If you think they are big enough switch the light to 12-12, but also know, that after you switch to 12-12, that they will double in size, or larger.
If you want smaller plants switch to 12-12 sooner, before they get to tall.
That's the best I can do, day or night does not matter, you flip the switch whenever they are the size you want, knowing that they will double in size, after you switch to 12-12.
Say you wake up and the light has been on 24hours a day for a month, and you go look at them and say ,damn they are big, set your timer for 12hours on, and 12 hours off, right then if you want too.
Darkness has nothin to do when to switch to 12-12, you switch to 12-12 whenever that they are large enough ,period, regardless if it's day or night.You go by the size of the plant.
Sorry, for not helping sooner, I've been at the Hospital all evening.
But I'm terribly sorry, to hear about your condition. And good luck with it.
But Nebula answers all these questions in her tutorials, And she is a hellava lot better, at describing how to do things than I am. So I would read her tutorials, in your spare time, and you always got help here on the forum, But expect, many different answers.
Have a nice evening 123.