Shinigami

Each shinigami in the fiction of Death Note posses a death note in which they write the names of those whose time it is to die. Shinigamies can see a clock above each person's head, once that clock reaches zero they are supposed to write down the name of that person, even if the shinigame does not write down the names of those who have no time left those perons will die anyway.
However when a shinigami writes someones name in the death note before their time is up their lifespan is prolonged, however when a shinigami prevents a person from dying (by saving them from an accident for example) their life comes to an end...