I was reading this question and it got me thinking about what a person's "original lifespan" really was.
Does the original lifespan mean the amount of time a person has from birth to death, ultimately caused by old age (e.g. the longest a person could live barring abnormal health problems or unusual circumstances (e.g. murders, accidents))?
What if the person contracts a lethal disease? Surely it would shorten their lifespan, but would it affect their original lifespan?
So, take for example: John has a lifespan of 93 years (as seen through Shinigami eyes). He works for the CIA, though, and at one point comes into contact with a high dose of radiation from a nuclear device. Doctors tell him he only has a couple days to live.
In this case, John's lifespan has been shortened from 93 years to 2 days. If he writes his own name in the Death Note, set to die from a different disease that takes years and years to develop, would he have effectively beaten radiation poisoning?
Note that the 23-day rule (XVII) would not apply here thanks to rule XVIII.