Actually, it's all over the place. During the main story, there are time skips, and then there are fillers and then the story, in the middle of the time skip, goes back to the original. Not to mention- the angst arc. Honestly, I don't think anyone knows. All I know is that they graduated, and now they're dating- much to the disappointment of Xiao mo
I say this with the most respect, but this story isn't that complicated and I don't always understand peoples confusion. It's actually really easy to follow! I think people don't sometimes pay close attention to context clues or have forgotten conversation that happened near the beginning of the story which help put a lot of the story time line together. Just my thoughts personally.
Sure! In present day chapter 461 - Sealed, He Tian opens a box that has been sealed away for a long time. He Tian finds everything that Little Mo had that connected him to He Tian sealed away in this box because it was so painful when He Tian left him. So your question is "when did He Tian?" well since in that box we see that Little Mo had He Tian's high school name tag and it says "Year 3, Class 5" this means He Tian made it to his final year of high school. So it is safe to say he left after graduating high school and went overseas to study abroad for university under the orders of his family.
To answer your second question, we have never got to see the boys graduate. The main story (which was back in the past and they were in middle school at the time) and now the present day (they are all working adults, so they are past the age of college students). If you have more questions, feel free to ask!

Can someone summarize the story when did he toan leave when did they graduate ive reread 4 times i still dont know