Arr I 1 I
Λέγεται δὴ Φίλιππος μὲν τελευτῆσαι ἐπὶ ἄρχοντος Πυθοδήλου Ἀθήνησι: παραλαβόντα δὲ τὴν βασιλείαν Ἀλέξανδρον, παῖδα ὄντα Φιλίππου, ἐς Πελοπόννησον παρελθεῖν: εἶναι δὲ τότε ἀμφὶ τὰ εἴκοσιν ἔτη Ἀλέξανδρον.
There is nothing here to indicate that this information is from Aristoboulos, if anything, legetai and the brevity of Arrian’s summary point more to an amalgm of his own from other than his main sources. We cannot say that Aristoboulos even covered this period as the Triballian narrative seems wholly Ptolemy, he is cited for casualties at I 2 vii and we know that the story of the Celtic embassy is his (he is cited as the author in Strabo VII 3 viii), the earliest story cited for Aristoboulos is that of Timokleia at Thebes. So it would be more probable that the ‘about twenty’ comes from Ptolemy. It is even more likely that the statement is Arrian’s based on Aristoboulos’ explicit statement of the length of the reign and Alexander’s life VII 28 i. Both figures coming direct from a contemporary courtier and the historian who preserves most of the detail of military organisation and belies any attempt to dismiss him as senile (not that I accuse you of doing that, but I did encounter it in my reading).
So to summarise the age at I 1 I is most likely Arrian’s approximation based on Aristoboulos’ accurate statement and not any confusion on the contemporary’s part.
Now we come to ‘by the Archon’; yes, it is true that ancient magistrates played fast and loose with the calendar but, and it is a big but these dates are a purely local affair for Athens, it is quite rare to find dual dates on inscriptions, other than the prytanny date or civil date. What is certain is that no Macedonian King would be able to refer to a future event using the ‘Archon date’ as even with Aristander by his side he could not predict the whim of an Athenian magistrate. The same goes for any historian, with the possible exception of the Atthidographers any month is almost certain to from the Festival Calendar which is the only one for which meaningful equivalents would exist in other states; though even then the months are unlikely to have been totally congruent due to vagaries in observing the exact date of the new moon or solstices.
That still leaves some discrepancies to be considered, notably Plutarch’s and Arrian’s different dates for Gaugamela. Now, in his Life of Camillus 19 iii
Now concerning "dies nefasti," or unlucky days, whether we must regard some as such, or whether Heracleitus was right in rebuking Hesiod for calling some days good and some bad, in his ignorance that the nature of every day is one and the same,— this question has been fully discussed elsewhere. 2 Still, even in what I am now writing, the mention of a few examples may not be amiss. To begin with, then, it was on the fifth day of the month of Hippodromius (which the Athenians call
Hecatombaeon) that the Boeotians won two illustrious victories which set the Greeks free: that at Leuctra, and that at Ceressus more than two hundred years earlier, when they conquered Lattamyas and the Thessalians. 3 Again, on the sixth day of the month of Boedromion the Greeks defeated the Persians at Marathon, on the third day at Plataea and Mycale together, and on the twenty-sixth day at Arbela. Moreover, it was about full moon of the same month that the Athenians won their sea-fight off Naxos, under the command of Chabrias, and about the twentieth, that at Salamis, as has been set forth in my treatise "On days." 4 Further, the month of Thargelion has clearly been a disastrous one for the Barbarians, for in that month the generals of the King were conquered by Alexander at the Granicus, and on the twenty-fourth of the month the Carthaginians were worsted by Timoleon off Sicily. On this day, too, of Thargelion, it appears that Ilium was taken, as Ephorus, Callisthenes, Damastes, and Phylarchus have stated. 5 Contrarywise, the month of Metageitnion (which the Boeotians call Panemus) has not been favourable to the Greeks. On the seventh of this month they were worsted by Antipater in the battle of Crannon, and utterly undone; before this they had fought Philip unsuccessfully at Chaeroneia on that day of the month; and in the same year, and on the same day of Metageitnion, Archidamus and his army, who had crossed into Italy, were cut to pieces by the Barbarians there. The Carthaginians also regard with fear the twenty-second of this month, because it has ever
brought upon them the worst and greatest of their misfortunes.
I am not unaware that, at about the time when the mysteries are celebrated, Thebes was razed to the ground for the second time by Alexander, and that afterwards the Athenians were forced to receive a Macedonian garrison on the twentieth of Boedromion, the very day on which they escort the mystic Iacchus forth in procession. 7 And likewise the Romans, on the self-same day, saw their army under Caepio destroyed by the Cimbri, and later, when Lucullus was their general, conquered Tigranes and the Armenians. Both King Attalus and Pompey the Great died on their own birth-days. In short, one can adduce many cases where the same times and seasons have brought opposite forces upon the same men.
I reproduce the bulk of the extract to demonstrate its nature; it is a collation of previously used, in ‘On Days’ (which is not extant) work.
In the Life of Alexander he has Plut Alx 31
Ἡ μὲν οὖν σελήνη τοῦ Βοηδρομιῶνος ἐξέλιπε περὶ τὴν τῶν μυστηρίων τῶν Ἀθήνησιν ἀρχήν,
The most important word being peri/about. It is more likely that this represents what Plutarch found in his source. Two things are immediately apparent, the date is not precise, as has been assumed and it is dated by an Athenian Festival which means the source is Greek, possibly Kallisthenes here as he is cited for the battle and as we can see from the Camillus passage he used Athenian months (unless Plutarch has been translating the originals to Athenian equivalents!).
This means that we have a range of Athenian dates. Vis-à-vis the eclipse of 20 Sept 331 Plutarch states that the eve of the battle was eleven days later, Arrian describes a four day march, another four day camp, a one day approach march and a day’s postponement, one day less but Arrian is not very clear and a day of reconnaissance seems to have been absorbed into the same day as the approach.
By the time he got to writing Camillus Plutarch has forgotten that peri and added twelve days to the start of the Mysteries on 14 Boedromion to get his 26th. Arrian must have taken the 19th, 20th or 21st as the date of the eclipse which would allow for the battle on 1st Pyanepsion. He has then combined the tradition that the battle was fought in the same month as the eclipse uncritically. Or he is translating a Macedonian date in Hyperbertaios as Pyanepsion, either is possible but that his source, presumably Aristoboulos or Ptolemy was using an’Archon date ‘ is not.
The Babylonian Astronomical Diaries (available at livius.com) give the date of the battle as 24th month VI Ululu which could imply that the Athenian year was two days behind or equally that Plutarch’s peri is really ‘just before’ ie. The 12th.
Hammond may think that Plutarch gets his date for Alexander’s birth from Timaeus but I see no evidence of it in the text the only source mentioned is Hegesias and he is accurate enough to take penalty kicks for England (soccer). Had Plutarch consulted Timaeus, whose work did cover Alexander his name ought to have appeared at chap 46 where he lists the sources for and against the visit of the Amazon Queen; it doesn’t. Hammond as you are well aware I am sure is frequently guilty of making bricks without straw and sometimes without clay either (witness his love of attributing passages to Diyllus whose two remaining fragments do not warrant the attributions).
Plutarch can be careless and is not particularly critical of his sources he is like Curtius, as good as the material he transmits.