For those that don't understand the timeline.
Early Chapters (1-50): These chapters depict the characters when they are 18-19 years old, after Jian Yi's return to Zhan Zheng Xi's life. Zhan Zheng Xi is in his first year of university, and Jian Yi is back in high school.
Flashback to Middle School (Chapters 51 onwards): The bulk of the current story acts as a flashback, showing the characters in their middle school days (around 15 years old), before Jian Yi's disappearance.
Christmas/Chinese New Year Specials (Various Chapters): These special chapters depict the characters at an older age (21-22), around the time He Tian returns and Zhan Zheng Xi is finishing university.
Post-Age 22 (Various Chapters): Some chapters and illustrations show the characters even older, when they are settled in their relationships.
Key takeaway: The story doesn't unfold strictly chronologically.
A more in depth look can be found here - https://zhanxixis.tumblr.com/19daystimeline
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