18th century for 1800s in not an uncommon error.
Except the article actually spells out eighteenth:
There are mysteries about Jack - a man going under his name seems to have a history stretching back to World War 2, or even earlier, to the Nineteenth century, the Eighteenth...
Truth is, Jack Harkness is immortal. A fixed point in time and space. The only one of his kind. For thousands of years, he has never been able to die.