Toki no Kairou

Main heroine Izumi is an ordinary student. But peaceful daily life starts to crumple with the visit of a boy in school uniform. Brutal murders and signs of supernatural forces pierce the reality.

At the same time the story of girl Saya begins. She is saved from death by a man, so she fulfills missions for him now with the current one being watching after Izumi. She witnesses human sacrifice ritual to resurrect the evil god. She has only five days to find the key point of the ritual and prevent it from happening.

[From vndbreview]