For those who don't understand this is my interpretation of the story. There are two different timelines. the present timeline is the one where Yeon-Ho is an adult and it's been 10 years since his friend died. The other timeline is where Jae-ha is still alive. The telephone in the room that Yeon-Ho finds is the only way to contact Jae-ha. When Yeon-ho confesses to Jae-ha about how he died and that he cannot go to the graduation and to meet him, Jae-ha does so. And in doing so Jae-ha was saved and that past timeline then became the present timeline. When Yeon-Ho goes out for a date and sees Jae-ha he runs towards him not realizing the truck and gets hit but survives, I say this because the author did not show a clear definition of him dying and also in the extra the author showed examples of Yeon-ho surviving never once dying. When we see Yeon-ho wake up as his younger self crying about a dream he had, that is the second timeline. The younger Yeon-ho memories were connected to his older self in the future which made it seem like a dream. I know that neither will die now because of the scene where Yeon-ho saved Jae-ha from getting hit by the car in their younger selves. They will always be together now.
Back in Time to You