So Haru was out for exactly the length of his dream... Rather than a dream, it seems more like Haru was reliving the past from when he was still alive, and the "present" we see in the manga is really just some kind of shared afterlife after Haru died in Master's "dream". There's also the butterfly in chapter 15.5. The butterfly represents both the soul and rebirth in many cultures, including in Japanese mythology. The post-apocalyptic world that they're in is the result of both of their desires. They're travelling the world together, going on walks together, and talking to each other, just as they wished.