The greater part of DTA is given over to the explanation of computistical matters. The history of the study of computus in western Europe in early medieval times has been set out elsewhere, and it is not within the scope of this work to rehearse it in detail. Suffice it to say here that the early Church celebrated Easter on a variety of dates in different regions, and the need for a universally agreed procedure for calculating the date of Easter, the central event of the liturgical year, became increasingly pressing as the centuries went past, and as Rome promoted its case to be recognised as the ultimate source of western ecclesiastical authority.

The first problem to be resolved in drawing up any calendar has always been that, for practical purposes, we need the calendar year to contain a whole number of days, yet the solar tropical year lasts 365.24 days. (A tropical year represents the time taken by the sun to return to a given point in the zodiac.) This results in the need to introduce leap years with an embolismic (intercalated) day every fourth year, so that four calendar years total 1461 (365¼ x 4) calendar days. Computists termed the intercalated day bissextus, as in the Julian calendar inherited from imperial Rome it resulted in two successive days being known as .vi. kalendas. Even this adjustment is too inexact over a period of centuries, and in modern times the Gregorian calendar has added the further refinement that the last year of a century (e.g. 1900, 2000) is only a leap year if it is divisible by 400. Once the calendar becomes out of kilter with astronomical reality, then it becomes more difficult to ensure that key events in the solar year, such as the equinoxes and solstices, are observed on the correct date. Bede subscribed to the tradition that the correct date on which to observe the vernal equinox was 21 March although, according to the calendar which he knew, it actually occurred in his time on about 17 March.

The equivalent ‘year’ of the moon, i.e. the time it takes to return to a given point in the zodiac, is about 27 days 8 hours. However, a more useful practical measure is the synodical lunar month or lunation, the period from one new moon to the next.