Rainbow Fireflies (2012) on Netflix

Plot summary

Yuta is twelve years old and in the sixth grade. His father died in a car crash one year ago. For the summer vacation Yuta travels to a deserted dam in the mountains, the spot he and his father liked to visit to catch beetles. There he meets a strange old man and offers him something to drink. Heavy rain makes Yuta slip and lose consciousness, and the old man saves him. He wakes to find himself thirty years in the past, in a village where a girl named Saeko takes him around. Saeko loves fireflies, and Yuta makes friends with a boy his own age.