I've derived Misa's lifespan.
This is a repost of a post I made on the TOWNL forums. This comes from a discussion of Misa's lifespan. Using the timeline in volume 13, along with the character page for Misa, I was able to derive her true lifespan. Mind you, I could be off by a few days, since we can't be sure of the exact dates of a few things, but I'm not off by much. The amount of years is correct.
If I made a mistake in my math, feel free to point it out...it's past midnight and I'm tired, so it's always possible that I screwed up.
For those of you in search of math, here we go. This is based on the timeline from How To Read 13, along with the assumption that Misa died at the end of her natural lifespan.
Misa was born on December 25, 1984.
Let's call Misa's original lifespan, the moment before the deal, "x". "x" is in days.
Misa had the shinigami eyes as of April 23, 2004. Since we don't know the precise dates, we'll say that was the day she made the deal (even though I know that's wrong by a week or two). As of then, her lifespan was "x/2".
She forfeited the Death Note some time around June 8, 2006, 48 days after we're saying she got them. Thus, her lifespan was then (x/2)-48.
On October 26, 2004, Misa regained the eyes and Rem killed L. I'm not entirely sure (I might be off by a couple of days), but it doesn't really matter. Thus her lifespan is [(x/2)-48)]/2.
Misa died on February 14, 2011. 6 years, 111 days had passed since her lifespan became [(x/2)-48)]/2.
I'll ignore leap years, since I don't want to worry about that. 6 times 365 is 2190. 2190 plus 111 is 2301.
Misa's original lifespan, on April 23, 2004, was 9300 days.
That's how many days she had lived up until the 23rd. That's 7054 days.
Misa was almost 20 at the time of the first trade, cutting her remaining lifespan to 12. She later cut it to 6, lived 6 years, and died.
Her lifespan was rather short.