Late Night Discount for Expressways in Japan

I have a question regarding late night discount for using expressway in Japan.
I have read on the website that late night discount is from 12:00 AM to 4:00 AM.
But my question is: if I enter to the expressway at 8:00 PM and exit at 4:00 AM then can I get the 30% discount, or I must enter at 12:00 AM and exit at 4:00 AM?

If you use a valid "ETC card" in a car equipped with "ETC",
you can get a late-night discount, no worry.
https://www.driveplaza.com/assets/img/etc/dis/etc_dis_night/img_night_...

https://en.driveplaza.com/expressways/etc.html

However, since idk when and which highway you will be using,
i cannot confirm whether there is a late-night discount or not, anyway, tho.

@Narumi vivio
Thank you very much for the detailed information.
I will go from Kyushu to Osaka in coming new year holidays. And I am expecting to use the highway at night time to get the discount.