Fred Donner's take on the Islamic era/calendar:
https://remmm.revues.org/7085The dating era that we usually call the “Islamic” or “hijra” era, beginning in the year 622 CE, provides another interesting example of an effort to project “Islamic” values back to the beginning of the Believers’ movement through creative re-naming. For the historian, there are two very striking facts about this era or dating system. The first fact is that we find it already in place at a very early time in the history of the community. There are quite a number of actual documents from the first “Islamic” century that provide an exact date as part of the document’s text—these include papyri, inscriptions, and, above all, thousands of coins minted by the leaders of the Believers’ movement or by their subordinates. Some of these documents are astonishingly early, including some early papyri dating to the year 22 (Bell, 1928). This fact obviously has very important implications for our understanding of the nature of the early Believers’ movement. In recent years some scholars have argued that Islam first emerged as the consequence not of an organized movement of some kind, but as the result of a series of fortuitous accidents: the collapse of the great empires, the conquest of parts of their territory by uncoordinated tribes of Arabian nomads, etc. (Sharon 1988; Nevo and Koren, 2003; Popp 2005). But it is difficult to defend the idea that the expansion of Muḥammad’s community was some kind of “accident,” that is, that is was the consequence of a haphazard series of unrelated events, in light of the existence of a uniform system of dating, which implies that there was some kind of central ideology underlying the movement. The mere existence of a single, new dating system does not tell us what this ideology may have been—perhaps it was related to apocalypticism?—but it does support the contention that the expansion was the result of some kind of coordinated movement.
The second striking fact about this era or dating system, however, is really just as revealing for the historian: it is that most of the early datings provide only the year (or day, month, and year), but do not provide the name of the era itself. That is, a document will say simply something like, “This was written in the year 55,” without any indication of the era—year 55 of what. Later Islamic tradition, of course, claims that these dates are in what we usually call the “hijra era,” that is, that the era commemorates the prophet Muḥammad’s hijra or emigration from Mecca to Medina in 622 CE. This event marked the moment when the Believers’ movement first assumed the status of an autonomous political community, and would indeed be a very appropriate starting-point for the community of Believers’ dating system. But the fact remains that none of the early documents—no single one—uses the term hijra to designate the era. A very few, however, do designate the era, but using another term: a papyrus in the Louvre, for example, refers to “the year 42 min qaḍâ’ al-mu’minîn”, “in the jurisdiction of the Believers” (Râġib, 2007), and a few others with this terminology are found in Vienna. This provides us with further documentary confirmation that the members of the community founded by Muḥammad at first thought of themselves as “Believers,” and that for a half-century at least they viewed their government as being a manifestation and application this concept of righteous Belief. But the question then becomes: when did the association of this era with the hijra, rather than with the Believers’ movement, begin? One simply does not find the term “years of the hijra” used in documents from the first “hijra” century, and the learned papyrologist Yûsuf Râghib has suggested to me that it may not really begin to appear in papyri until the third century “of the hijra” (personal communication, 2008). It is all the more striking that the era is not linked to the word hijra until so late, since it has been suggested that the term hijra was used in the early community to refer to military service (Crone, 1994); this is presumably why the Believers are first referred to by some seventh-century Greek and Syriac sources as hagarenoi and mhaggrâyē, derived from the Arabic muhâjirûn, “those who make hijra.” It seems, then, that in this case, too, we have an example of a later attempt to associate a well-established practice—the practice of dating—with the prophet Muḥammad (through his hijra). Renaming the era as that of the hijra enhanced the legitimacy of the practice of dating itself, and of the community and state that was associated with the practice, in terms that were “Islamic”—that is, tied to the prophet and Qur’ân—but in this case, perhaps just as important, the renaming helped efface the memory of the more ecumenical nature of the early community as a “Believers’ movement.”