Harukanaru Toki no Naka de 5

In the year 2020, the protagonist and her childhood friends prepare for their Christmas break. As they board an airplane to return to Japan, they are sucked into the parallel world. This world is experiencing the turbulent end of the Tokugawa shogunate in December 1863. Using a magical hourglass, she gains the unsteady power to travel to both worlds. Although she and her companions flicker in and out of the parallel world, her adventures within this world span a total of three years.

Initially, the protagonist desires to save the suffering Four Gods from the parallel world's warring factions in order to save her mysteriously ravaged home world. As time passes, she learns that both worlds are being merged to create another world. Hoping to prevent the forceful deaths of two worlds, she willingly sacrifices fragments of her own life force to protect those dear to her.

[from Koei Wikia]