Is Nijiiro Hotaru: Eien no Natsuyasumi hentai?
English title: Rainbow Fireflies
No
Rated G (All Ages) on MyAnimeList and not tagged as Hentai.

Movie · 2012 · 1 episodes · G - All Ages
Fantasy, Romance, Time Travel
Yuuta was 12-year-old boy, who had lost his father in the traffic accident one year ago. In the summer vacation, he visited a deserted dam deep in the mountains, where he had a good time with his father before. Suddenly a thunder storm occurred and he slipped on the ground. He lost consciousness and woke up to find a girl and an unfamiliar village. He time-traveled 30 years and reached a village, which sank at the bottom of the dam. This is Yuuta's precious memory of "another" summer vacation.
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