Most of what you see, in this manhua, is THE PAST, with 4 of them attending middle school, wearing gym tracksuits.
Scenes in PRESENT < Where Zhan Zheng Xi is a College student > are very few, for ex. :
-the first chapters until the very long flashback < here on Mangago flashback starts from Ch 51 and what we read now - with very few exceptions - is still inside the flashback>
- Christmas and other holidays < ex. Ch 224 Where we discover that even Mo and He Tian had separated, who knows for how much, and they are seen again for the first time >
- very few others in the middle, like those in which they roll in the snow <Here on Mangago Ch 68> and usually correspond to special days like Christmas, New Year, Valentine's Day and so on ...
The only thing we know about what happened in the middle is that Jian Yi says - but will it then be true? - that he had disappeared because he was taken to a desert island by his father's men <Here on mangago Ch46>
Jian and Xi don't go to high school together because Jian disappears on the second day of the first year and then reappears when Xi is at College. In fact, Jian tries to study to recover the lost school years and go to the same college of Xi.
I hope I helped you a bit!
Sorry for my English, I'm not a native speacker <3
Every time I remember that what we read now is the past, and that in a very short time our 4 babies will all be divided, I get so sad.
P.s. If you think this is a Spoiler, you should re-read 19 days all over again