Right Here Waiting for You.

A middle-aged man time travels to the past to become a teacher to his 17-year-old classmates. Hao Hui Gui is a 36-year-old high school teacher who is still single. During a gathering, he gets into a huge disagreement with everyone present and even gets into a fistfight with his best friend. Upon returning home, he mindlessly flips through his personal diary from 20 years ago and starts to recall many memories from the past. He falls asleep eventually and meets with his younger self in his dreams.