How do you co-ordinate communities effectively without modern technology?
It is a well-repeated factlet that time only became properly uniform with canal and train timetables during the Industrial Revolution. Technology allowed it to be the same 10:45am in London as in Glasgow.
Uniformity of time in Antiquity and the Middle Ages was still pretty important. Some of the earliest extant canon law – the rules governing the Church – ordered that Easter was to be celebrated at the same time everywhere in the world. Okay so getting the date agreed was not quite the same feat as getting the minutes and seconds in sync, but it was still impressive.

Before even the debates about co-ordinating Easter dates, the Roman Empire had been shaped by its civic rhythms. Roman calendars are full of information about days you can start legal action and days you cannot, plus civic and religious festivals. The calendars that have survived tend to be those carved in marble or stone, but we know that many people owned their own papyrus calendars too. It was important to know what was supposed to be going on and when one needed to engage with different social activities. No surprise there.
In the early Middle Ages, as Roman imperial structures weakened, the Church took a lead in co-ordinating social action. The centrality of Easter to the calendar was crucial. Partly, this was first because Easter kept moving, while determining a significant portion of the rest of the liturgical calendar. In 541 in Orléans it was decreed that ‘the people’ had to be told in church at Epiphany (6 January) when Easter was going to be well in advance, presumably so they could prepare and be ready for Lent. It helped to stop disagreements.
The problem was that there was more than one way to calculate Easter. The lunar and solar years are awkwardly different in length, and it did not help that there were different models of lunar cycles in circulation. Most famously this led to a clash in Northumbria in 664 at the Synod of Whitby. He had followed an Easter table in use in Iona, now in Scotland, which was often as much as a month out of sync with the Gallic Easter table used by his wife. This was not good for marital harmony, given the things you are not supposed to do during Lent. It was not good for social cohesion either, if different groups were arguing about the true Easter date. So Oswiu renounced his old Easter observances and embraced the ‘Roman’ one. Unfortunately, even this decision seems to have been a bit of a fudge, as there were two different tables in circulation which had some sort of Roman backing – one by Victorius of Aquitaine and one by Dionysius Exiguus. (Bede tells the story in his Ecclesiastical History III. 25 as if it were a straight victory for Dionysius, but in On the Reckoning of Time he lets slip that things were more complicated).
In the Frankish world, it is clear that communities used Victorius or aspects of his work as late as the 760s, while Dionysius’s work was in use from at least the 690s in the circles of St Willibrord and Pippin II. The two tables did not often disagree, and only ever by a week. Still, this was enough to cause tensions, reported in the Historia vel gesta Francorum (= the Continuation of the Chronicle of Fredegar) for 740 and noted in a list of topics for discussion at the Council of Soissons in 744. A new consensus based around Dionysius, and later around the work of Bede, soon developed.
So how was time co-ordinated now? Let’s finish with one concrete example: the famous manuscript of St Willibrord’s, which contains his own calendar. You can look at this online here. It is fascinating because it contains a copy of a martyrology, listing saints days in calendrical order; then there is a sparser calendar with saints’ days and memorial notices about recently deceased friends; and finally, there are the Easter tables, extended in various bursts from 684 (in Ireland) to 797 (on the continent). This was a practical workbook, likely kept at the church at Echternach to assist with co-ordinating the year. Many more calendars-plus-tables would be produced around the Frankish world, some with treatises on astronomy for the technically-minded, some with collections of canon law and sermons for priests. In fact, calendars and tables are some of our most common sources for the period after biblical materials and landholding records. Charlemagne even ordered that people learn about them in schools from 789 onwards.
And there we have it: Western Europe was co-ordinated in time through an international network of religious institutions and by encouragement of a particularly powerful royal figure, largely by having a wide distribution of practical books.
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For more on this subject, a decent introductory read is Georges Declercq’s Anno Domini: The Origin of the Christian Era or Arno Borst’s The Ordering of Time. But Immo Warntjes’s research is shifting the paradigms for our understanding of the calendar in the early Middle Ages, so this may well change.