Been most remiss - the "real world" has its demands...
The issue at heart is the dating method employed by Diodorus. In a world of multiple calendars commencing at disparate times, Diodorus settled on a chronological framework of Olympiads, Roman magistracies and Athenian archon years. The latter, for our purposes, are what matter for it is how Diodorus delineated successive years and it is his handling of these fixed datums which is the crucial element in the debate.
For Books 18-20, the source which Diodorus followed “arranged his history by campaigning seasons, equivalent to the years of our calendar, clearly marking the end of each season by indicating the winter quarters of the various armies” as Russell Greer noted in his introduction to Volume 9 of the Loeb. Greer went on to posit that Diodorus “gives under each archon all the events of the year during which he took office”. This was something Smith (
AJP, Vol. 82, No. 3, 1961, pp. 283-290) picked up on and demonstrated though, because the “low” was the “accepted” chronology of the time and he used only select examples (316, 308, 307 and 302), his argument was largely rejected. Recently Meeus (
Phoenix, vol. 66, 2012, 1–2, pp 74-96) undertook a full appraisal of Book 19 and confirmed the prescience of both Greer and Smith. The argument is detailed and fulsome and I will not detail it here as the paper is available for those who wish it. The point should be constantly in mind as one reads either argument.
Anson adduces much evidence in his essay to support his position though the one, somewhat, equivocal piece is the flight of Seleukos from Babylon (316 high; 315 low). This is understandable for, as has been mentioned, this is a crucial chronological reference and an insurmountable impediment to the low since Stylianou demonstrated back in 1993 that it occurred in 316 (“The Pax Macedonica and the Freedom of the Greeks of Asia [with an Appendix on the Chronology of the Years 323-301.”]
Epeteris tou Kentrou Epistemonikon Ereunon, Annual of the Cyprus Research Centre, 20:1-84). Anson is at pains to minimise this by claiming that “it is reasonably clear” that Seleukos fled Babylon in the late spring of the given year. This allows him claim that under either chronology three and a half or four and a half (high and ow respectively) was the actuality. Conveniently Anson accepts the position of Smith and Meeus here in that Diodorus has recorded the events of spring under the archon for 315/16. More to the point, Anson is specific with Diodorus’ timing in other matters. For example, Anson is as certain as Diodorus (and his source) that Perdikkas ruled for three years (
AJP, Vol. 107, No. 2 [Summer, 1986], p 212):
In the first place he [Diodorus] states (18.36.7) that Perdiccas' death occurred after he had ruled for three years. This would place his death in 320, his regency having begun in 323. Since these exact time references do not derive ultimately from Diodorus himself, but from his source Hieronymus of Cardia, who is generally considered to have been a careful and accurate historian, this particular chronological reference should be considered reliable.
Just as he is certain that Diodorus (via Hieronymus) is accurate in Arrhidaios’ taking almost exactly two years to complete Alexander’s funeral cortege. Just as he is certain Diodorus (and his ever-reliable source, Hieronymus) is correct with the rule of Philip Arrhidaios. Yet, here (19.91.2) Diodorus (and his ever-reliable source) have got it all ballsed up. Just how could it come to this? A good question and one I might address when next I get some minutes from the “real world”.
All the papers referenced above are, of course, available via PM.