There has been quite a lot of confusion with respect to the anniversary of the Battle of Marathon (490 BC) which was fought approximately 2500 years ago. Many organizations have proclaimed 2010 as the date in which it should be celebrated...and at first glance, it does appear to be correct. However, upon closer scrutiny, it has been determined that the 2500th anniversary of the Battle of Marathon should be observed in 2011. To illustrate this point and to provide more clarity, the following example will be used - The calendar counts down in the following manner: 3 BC, 2 BC , 1 BC, 1 AD, 2 AD, etc., therefore, based on this calculation which accounts for the fact that there is no year zero: 490 BC - 1 BC = 489 +2011 = 2500.
As far as the exact dating of the Battle of Marathon, that is much more difficult to ascertain. Authors and scholars have different theories as to when it was fought, which is understandable since it occurred close to 2500 years ago. Therefore, if the exact dating of the Battle of Thermopylae (480 BC) can't be agreed upon, it shouldn't come as a surprise that there isn't a consensus with Marathon, which preceded Thermopylae by a decade. Fortunately, Herodotus mentioned the ancient Olympic Games, which coincided with the years these battles were fought in. As a result, it has been possible to reconstruct the dates.
Texas State University faculty Donald W. Olson and Russell S. Doescher, who are physics professors along with Marilynn S. Olson who teaches English, in their published article entitled The Moon and the Marathon which appeared in the September 2004 issue of 'Sky & Telescope' have an alternate theory which differs from what has been accepted by the majority of scholars. They have managed to pinpoint the date with such specificity, that readers who are interested in the Greco-Persian Wars should read their conclusions along with the supporting analyses.
For the sake of brevity, the main points will be summarized below, however, I urge you to read the complete article since it is quite fascinating.
Olson, Doescher & Olson's research is based upon the Spartan calendar instead of the Athenian's, which many in the academic community have accepted as the standard. Herodotus recorded that the Spartans did not arrive at Marathon until after the battle had been fought, therefore, there is quite a bit of validity in using the Spartan version, especially since the city-state of Sparta observed the Karneian Festival which prohibited them from marching during one of their most religious holidays.
Scholars have also doubted the veracity of the tale that the runner Philippides (Pheidippides) died after having delivered the message "Nike" or a derivative of the word (Victory). Herodotus recorded that Philippides ran from Athens to Sparta (approximately 150 miles) requesting the Spartans' help prior to the battle. However, there was no mention by 'The Father of History' that the herald ran from the battlefield to Athens after the Athenians and Plataeans had emerged victorious over the Persians. The renowned historian and biographer Plutarch noted that it was Thersippos of Eroiades, who ran from Marathon to Athens and expired upon the announcement that the Greeks won.
Whether it was Philippides/Pheidippides or Thersippos, the date of the battle has been cited by many authors as occurring in September. However, the contention by Olson, Doescher & Olson is that the date of August 12th is when the Battle of Marathon was fought, which is one month earlier than what has been believed to be correct. Further strengthening their argument is that September is much cooler at this time in Greece, therefore, the August heat of 2500 years ago, combined with effort expended during the fighting, and the distance of approximately 26 miles from the battlefield to Athens all could have contributed to the messenger's death.
Whether the Battle of Marathon was fought on August 12th or September 12th, or any other date for that matter, its dating is more a matter of historical interpretation than that of the calendar.