Although the Mayan calendar goes deep into the past, the early events it records are legendary. The earliest historical date known is 28 May 585 BC, the battle between the Medes and the Lydians that was interrupted by an eclipse predicted by Thales of Miletus.
I would have said:
The earliest historical event that can be accurately dated was the battle between the Medes and the Lydians on 28 May 585 BC. We know the exact date because it was interrupted by a solar eclipse that Thales of Miletus predicted.