This article examines Arrian’s decision to date events in the Anabasis and Indica by eponymous archon of Athens and Athenian month. In the past, scholars have only been concerned with whether the dates are correct rather than with why Arrian is using them in the first place. His sources Ptolemy and Aristoboulus did not use this dating system, so Arrian must have converted the dates he found in his sources to this format, and he must have done so for a purpose. It is argued that the key to the schema is Arrian’s use of an archon date to mark Alexander’s crossing of the Euphrates at Thapsacus: this is an unimportant event in Alexander’s anabasis, but a crucial turning point in Xenophon’s. The archon dates thus structure a comparison between the two anabaseis, emphasizing how much greater Alexander’s was by comparison with Xenophon’s in line with his comments on the subject in the Second Preface. Arrian used Athenian archon dates for this purpose to indicate his own familiarity with Athens without having to explicitly state it. This is indicative of how, again in line with his statement in the Second Preface, he preferred to intimate details of his biography to his readers rather than state them openly.