The medieval Middle East employed a surfeit of calendars which can bewilder the unwary researcher, but sometimes the multiplicity of systems for identifying time can in fact be helpful. Scribes often failed to identify the date they were writing more precisely than by giving the year, but if they provided the year in more than one calendar, it can help narrow down the time in which they wrote (assuming they were accurate in their conversion). Sebastian Brock created a list of medieval Syriac scribes who provided dates in the Hijri calendar, and he notes both when scribes employed additional calendars (up to six!) and when their conversions between calendars were mistaken. I recently had occasion to use this trick for a very interesting fifteenth-century text.
Adam McCollum discovered a brief fifteenth-century chronology of the world which gives the years of various epochs since the first human, Adam.  The scribe’s date of composition is not given more precisely than the year 1770 A.G., which corresponds to the year between 1 October 1458 and 30 September 1459. But we can most likely narrow it down further than that.
The last period of the chronology, that which the scribe identified as present, was the period of “Arab rule” which began, according to the text, 862 years earlier. This looks like a simple error: the Arab conquests did not begin anywhere near as early as 597 CE. But different calendars might be used for different purposes, and the main calendar used to compute time since the rise of Islam is the Hijri calendar. As it happens, 862 Hijri corresponds to the lunar year from mid-November 1457 to mid-November 1458 overlapping with 1770 A.G. by only about six weeks in the early autumn. This co-occurrence of 862 Hijri and 1770 A.G. suggests, unless the Hijri date is off by one, that in fact the chronology was composed in October or the first half of November of 1458. The use of two calendars enables us to know not only the year, but the most likely season of the year when this text was composed.
It is decidedly odd that the scribe included the number of Hijri (lunar) years and simply added them to the solar years his chronology uses elsewhere. It’s possible he was not familiar enough with the Hijri calendar to realize that its years were significantly shorter than solar years. (Consequently, at about the time Americans start wishing they were younger, their age in the Hijri calendar is higher than in the Gregorian calendar!) The scribe may even have simply asked a Muslim neighbor how many years ago the Arabs began to rule, to which the neighbor responded with the Hijri year, the number of lunar years since Muhammad left Mecca for Medina and set up the first Muslim state, according to tradition. The use of shorter years since the rise of Islam has misled this scribe into portraying the Arabs as having ruled earlier than they in fact did. If the advantage of multiple calendars is a potential gain in precision, the drawback is the increase in confusion, both for the people who lived with those various calendars, and for those of us attempting today to navigate that world from afar.
 Sebastian P. Brock, “The Use of Hijra Dating in Syriac Manuscripts: A Preliminary Investigation,” in Redefining Christian Identity : Cultural Interaction in the Middle East since the Rise of Islam, ed. Jan J. van Ginkel, Heleen L. Murre-van den Berg, and Theo M. van Lint, Orientalia Lovaniensia Analecta 134 (Leuven: Uitgeverij Peeters, 2004), 275–290.