Possibly the slowest-paced movie I've ever seen in my life, I nevertheless found it utterly mesmerizing. Makoto Shinkai (who produced, directed, and wrote) has a way with images and the philosophy of teenage love and angst that's just amazing. The images are a very strange blend of Anime with occasional touches of photorealism, and manage to make things like train schedules and power lines look stunningly beautiful (and sunsets and cherry blossoms even more beautiful than that). The movie is mostly about Takaki Tōno and to a somewhat lesser extent his middle school companion Akari Shinohara. The movie is divided into three segments - when Takaki is about 12, 16, and 22. The first 20 minutes consists of Takaki riding a train, with his voice-over and flashbacks about the girl he's going to meet. And the train delays - we spend a lot of time watching and hearing about train delays.
I suppose the movie is about longing and what-could-have-been. Philosophical, down-beat, and short (only about 60 minutes), it's not for everyone, but fans of the genre should absolutely see this as it's very good and it's already clear in 2017 that Shinkai is a rising star.