Vanishing Time: A Boy Who Returned
가려진 시간 (original)
Four young kids, including Sung-Min and Soo-Rin, take a trip to the mountain together. They plan to sneak into a restricted area to watch a planned explosion. The next day, Soo-Rin is the only one found. A few days later, Sung-Min appears again, but he is not the same 13-year-old boy. Sung-Min is now adult.
Starring Gang Dong-won, Shin Eun-soo, Lee Hyo-je, Kim Hee-won, Kwon Hae-hyo, Kim Dan-yul, Um Tae-goo, Park Jong-hwan, Moon So-ri, Ahn Ji-ho
Watch "Vanishing Time: A Boy Who Returned" from streaming services Tubi TV, Roku Channel released in 2016. It's a drama and fantasy movie, and has a rating of 7.2 out of 10 from Imdb which is usually fine for this kind of movie, and is currently available to watch for free.