Honestly, I think to assume he hadn't tried it (assuming his pathology was constant) would be in error itself. It would be far more correct to assume that he had come to this idea at some point in the time of his initial reception othe Death Note, only to come to the conclusion that he would never be able to confirm his theory, as the person would die AFTER his death. You could also infer that were he able to enact such a cause of death, the most accurate a test would be to have the person dies moments BEFORE, but once again taking Light's personality into account it wouldn't be likely for him to take an experiment that far, and choose to work with proven abilities.Graustein said:I don't think Light is ever shown using the Death Note to indicate a time expressed in terms other than a specific clock time on a specific date. Knowing Light, he would definitely have attempted it - the uses for such a function would be too great for Light to not try it - and it would've been used in at least one of his schemes. Since he's not shown using it, I'm inferring that it's not actually possible to do that. It probably came up during his experimentation phase early on, and simply wasn't mentioned in the series proper. While it's just speculation on my part, the idea that someone as meticulous as Light didn't at least try it and, were he successful, use it extensively is so unlikely that it makes me assume it's not, in fact, possible.
I don't recall that being one of the rules mentioned during the anime, either during the story or the bumps midway each episode. I do recall seeing the twenty three day limit in regards to killing by a disease. If it was mentioned in the manga, live-action films, or other media, and is considered canon then I retract that statement. As far as I looked into it, the only limits are in regards to the nature of death (Six minutes and forty seconds), and the nineteen day limit to write a vitcim's name in the Death Note after writing down a cause of death.Chaotic Neutral said:No, the Death Note has to kill its target within 23 days of writing the name down, so you can't abuse it to make someone virtually immortal ("I will die painlessly five minutes after the heat death of the universe, having been totally healthy up until that point"). One of The Rules(tm).
It's been a while since I last read/watched the manga/anime. Please forgive me if I've made any mistake.