system
Given the following post, respond as if you're commenting on Reddit:
human
[Death Note] How far can a person's life be extended using a Death Note?

For a Shingami, killing a person before they are fated to extends the Shinigami's life, and extending a human's life instantly killed them.  But if you, a human, write in the Death Note, you are not subjected to those rules, and can freely alter a person's fated death (at the price of pure nothingness instead of an afterlife).  

How far can a person's life be extended using a Death Note, then?  Could you technically write your own name, and date it 60 years from now?  Or is extending a person's life like dividing by zero - they have 2 fated deaths listed, and the first one to happen is the one that kills them?
gpt
If you are scheduled to die in 50 years, and you write your name in to die in 80 years, you will die of a heart attack in about 6 minutes. This also applies to if you write your own name and then attempt to test your own immortality. If you were going to put yourself in a situation that would result in your death, or would be in that situation, either you luckily manage to avoid death, or heart attack. However, if you are scheduled to be murdered, and you kill the murderer, you can safely extend your own life. It basically depends on the circumstances of your death.