When did the Athenians march out to meet the Persians?

In an attempt to sort out the confused chronology of events before the battle of Marathon, J. A. R. Munro, in the Cambridge Ancient History, vol. iv, maintains that the march must have taken place whilst Eretria was still in a state of siege, and was designed to save that city.

His supposition is based on a statement in Aristotle's Rhetoric, iii. 10, 1411 a 10, where Cephisodotus παρακαλν ποτε τούς 'Aϑηναíους εìς Eὒβοιαν ἐπισιτισαμένους ἒϕη εīν ἐξιἑναι τò Mιλτιἁλου Ψἡϕιαμα. Munro points out that the only possible context for this decree of Miltiades is the Persian investment of Eretria; he then goes on to presume that the decree was implemented, and that the Athenians set out from Athens some time before the date given by Herodotus for their march. To account for their non-arrival at Oropus he suggests that the Persian forces divided, and by an early invasion of Attica diverted the Greek army from its original intention.

A ψἡϕιαμα was a most binding enactment, and could only be can-celled by another ψἡϕιαμα. Because there is no record of such a cancellation, Munro seems to think that the decree must have been carried out. But both Herodotus (vi. 102 seq.) and Plato (Menexenus, 240 b) expressly declare that only after the subjugation of Eretria and the landing at Marathon did the Athenians move from the city.

Munro's difficulty about the ψἡϕιαμα could surely be better dealt with if we suppose that the force of the decree was completely invalidated by the fall of Eretria.