It was revealed in Death Bringer, that in War happened around 1850, so Skulduggery could have died as early as then. Though I'm not sure how the exact year comes along. Derek has confirmed the exact date as his own birthday, though. —Resident Devil 10:05, February 7, 2013 (UTC)
It should probably be noted though that while his death year has never been revealed in the books, anything 19th century is highly unlikely, considering that we do know he's over 400 years old and that he said, in the first book, “Compared to how long I’ve been like this, I was only technically alive for a blink of an eye. I can’t really remember enough about having a beating heart in my chest to miss it.” - this makes it very unlikely he was alive for 250 years and has been a skeleton for 150 years.
Derek Landy has also said on Bebo and confirmed on his blog that Skulduggery was "in his sixties" when he died, so it would seem most likely that he died around the 1660s. —Kribu (talk) 10:38, February 7, 2013 (UTC)