Harukanaru Toki no Naka de 5

In 2020, the protagonist and her childhood friends are on an airplane heading home for Christmas break when they are pulled into a parallel world. The year there is December 1863 — the turbulent final months of the Tokugawa shogunate. A magical hourglass gives her an unstable ability to cross between worlds, though she cannot control when she and her companions flicker between them. Her goal is to relieve the Four Gods suffering under the parallel world's warring factions, believing this will also save her own world, which she finds ravaged on her returns. In time she learns the truth: both worlds are being merged into a third. Rather than allow two worlds to end violently, she begins sacrificing fragments of her own life to protect those she has come to love.