Battle of Halys - May 28, 585 BC. The date is known because the battle was interrupted by a solar eclipse, which enabled astronomers to exactly calculate it.

What do you mean, "confirmed"? I can confirm for you right now that 10th June 1,000,001 BCE definitely happened. Or do you mean the oldest date with an "event" attached to it? In that case we know that various prehistoric monuments around the world were constructed to take advantage of astronomical events that can be dated precisely, so for example there was almost certainly some sort of event on the Summer Solstice shortly after Stonehenge was built, let's say 21st June 2500 BCE (but someone less cautious about when Stonehenge was built could beat me and pick an earlier date.) Or does it only count if somebody wrote about it? (And the writing wasn't lost?)

My point being: like almost any question of "the oldest X" this comes down to "define X" and there is never one satisfactory way of doing that. Therefore the answers you'll get to this question are going to be a range of trivia about things that happened a very long time ago which may or may not be accurate and may or may not be interesting. That isn't real history.

I posted an answer to a similar question awhile back. It's April 21, 899 BC http://www.reddit.com/r/AskHistorians/comments/1yrc31/what_is_the_oldest_historical_event_that_we/cfn57dc

That's almost 3000 years ago!