Comet C/2006 P1 (McNaught)
In 2006 Comet McNaught zipped into the solar system travelling at over 90,000 mph, passing within Earth's orbital distance from the Sun (0.17 AU), on a 762,000-year orbit that takes it out as far as 8341 AU from the Sun. It is known as the Great Comet of 2007. It was the brightest comet in 40 years, easily visible to the naked eye for observers in the southern hemisphere.
This is a perfect example of a dangerous near-earth object IMO.
It was discovered on August 7, 2006.
It reached perihelion on January 12, 2007.
That's FIVE MONTHS advance warning for unknown KBOs and Oort Cloud objects with million-year orbits.