Is there a time skip? College? When did you get back?

The start and some special chapters are in the "present". The main story is the "past", like a biiig flashback.

Omg realllllly? I never knew. Is that why theres "One Day" on the chapters?

I never thought about it, but makes sense too. The title is "19 days" I think it's like "the days we were 19yo". Also at the start we see jiyan visiting xixi in his apartment alone, and in the past he lived with his family, that's another point