I think there was a pretty short limit on how far in the future you could write someone’s death.

23 days. You had 40 seconds to write a cause of death and 6:40 to write the details, and could even pre-write the details and add the name(s) afterward but the Death Note could not effect anything that occurred more than 23 days in the future, with the exception of dying by a disease that would take longer than that to progress and not writing a time (in which case they develop the disease but die when/how the disease would take them). The real problem is that you also can't use the Death Note to directly extend someone's life by writing that they die after they normally would - if you try they just die however they would have died if you hadn't used the note. You can however use the Death Note to kill someone who is a threat to another, indirectly saving their life.