In our everyday world, a calendar is generally something that you hang on the wall, and by which you count off the days until some birthday, a vacation, or the next public holiday. A calendar is a much bigger concept than this, though, and it affects both genealogists and historians in ways that we don’t like to think about since they complicate our worldview.
So what is a calendar? A calendar is a mechanism by which dates are reckoned in a given culture. Historically, that meant that it allowed the passing of days to be recorded and so the return of the seasons or astronomical phenomena to be predicted.
There are six principal calendars in current use: Gregorian, Jewish, Islamic, Indian, Chinese, and Julian, but a list of many historical calendars can be found at: List of Calendars. If we encounter a source with a date expressed according to one of these calendars then how should we represent it? Before I try and answer that, I want to give a small tour to illustrate how complex the subject is.
Calendars can be based on natural cycles, such as astronomical events, or contrived (man-made) cycles. The main astronomical systems are:
Lunar. Calendars based on the counting of lunations: cycles of the phases of the moon. The average lunation is now known to be 29.530589 days, but historically it wasn’t known to that level of precision. Addition of an extra day, to get things back in step, might have been done on ad hoc basis rather than according to some rigid rule. Other problems — other than the natural fluctuations, and observing phase changes in bad weather — include the fact that the first instant of a new phase depends on both the latitude and longitude of the observer. The Islamic calendar is the only modern-day example.
Lunisolar. Calendars where the cycles of the moon and of the year (i.e. seasons) were combined and extra months occasionally added to keep them synchronised — sometimes relying on an arbitrary decision by a local priest rather than a formulaic approach. The Jewish, Chinese, Japanese, and Indian calendars are examples.
Solar. Calendars based entirely on the cycle of the sun, and abandoning those of the moon. Although our modern Gregorian calendar has months, these are not tied to the phases of the moon and so it is a solar calendar rather than a lunisolar one. Other examples include the Egyptian calendar, which had a fixed 365-day so-called “wandering” year, and the Julian calendar which had a fixed 365.25-day year.
The root of many problems with calendars based on natural cycles is that there is no fixed integral relationship between the periods of astronomical events such as the rotation of the earth about its axis (a day), the phases of the moon (a month), and the rotation of the earth about the sun (a year): their relationships are both fractional and continually varying. In fact, we cannot even say that the length of a day now is the same as it was at some point in prehistory because there was no common yardstick by which to directly compare them. We can extrapolate our understanding of the motions within our solar system back to ancient times but they’re the predictable motions. If some asteroid had once passed close by the earth then it could have had a significant effect on the mean solar day. We’re currently aware of these fractional relationships by virtue of the leap-year where an extra day is inserted into our calendar every four years — except if the year is a multiple of 100, and it is not a multiple of 400. This process of adding a day (in general: intercalation) is simply trying to keep our notions of a day and a year in step.
Although several instances of contrived cycles have once existed, our most familiar modern-day example is the seven-day week.
The years themselves may be counted from the beginning of some reign or from some important or regular cultural event. Years counted according to the reign of some sovereign, monarch, or pope are termed regnal years. Even now, Acts of Parliament, in England, might be dated such as 3 Elizabeth II, meaning: third year of the reign of Elizabeth II. In ancient Greece, years were sometimes numbered according to the Olympiad: the four-yearly period between their successive games. The Japanese nengo system counts years according to a number of eras, originally determined by court officials but later determined by the accession of emperors (similar to the old Chinese dynasties). Our own modern year numbering is according to the Christian Era (AD/BC), also known as the Common Era (CE/BCE), although this has only a single epoch. Years in the French Republican calendar began with the Republican Era: the first year of the republic. Years may sometimes be named rather than numbered, as with Roman ones based on the name of a consul, or Greek ones based on the name of an archon. Knowledge of the associated epochs — the starting points for the counting — in some alternative calendar system is therefore a prerequisite for accurate conversion to it. This knowledge becomes less known as the importance of the epoch becomes more minor. In England, the records of manors held by the church sometimes used Episcopal or abbatial years. Even though the sequence of bishops or abbots is generally known, their exact dates may not be — especially in the case of minor abbots who did not sit in the House of Lords.
What the majority of the world uses today is known as the Gregorian calendar, named after Pope Gregory XIII who introduced it via a papal bull in 1582 as a replacement for the Julian calendar. The Julian calendar used a fixed year of 365.25 days, but the average tropical year is more like 365.24219 days, and that meant that events were drifting very slightly behind each year. A major problem was that Easter was defined by the First Council of Nicaea (AD 325) in relation to the spring equinox, and after nearly 13 centuries of the overly-long year its date had drifted backwards by about 10 days of where it used to be. As well as setting a new average calendar year of 365.2425 days, the Gregorian calendar removed 10 days in order to move Easter back to where it was before.
I don’t want to dwell too long on this Julian-to-Gregorian transition during this particular post (I’ll cover that another time) as there are many other calendar systems. I want to illustrate the complexities of those calendar systems, and explain how converting between them is not an exact science. If we allow our modern Gregorian calendar to be used in a proleptic fashion, where it can be used for dates prior to its invention, then we find that there is some uncertainty in translating dates from those other systems to our modern system.
The Julian calendar was introduced by Julius Caesar in 46 BC as a reform of the earlier Roman calendar, supposedly introduced when Rome was founded by Romulus in about 753 BC. The Roman calendar was a lunar calendar with 10 months (December being the 10th month) and a year of 304 days. This was unworkable for farmers who needed to be more aware of the approaching seasons, and so Numa Pompilius (the second king of Rome), in about 713 BC, introduced two more months, thus making the year 354 days (12 x 29.5 days), although an extra day was added to make it 355 for superstitious reasons. This still drifted with respect to the solar year and so several schemes were devised to try and improve it using intercalary days or months. However, the decision of when and how-much were usually in the hands of the priests who shamelessly misused that power — when they hadn’t neglected the actual need — for political reasons (e.g. changing the term of someone’s office) or financial advantage (e.g. taxes, rents). When the Julian calendar was introduced, there were so many corrections to put things back in order that 46 BC was a year of some 445 days, often referred to as “the year of confusion”. In effect, there are many calendar variations here, and some rather irregular intercalations.
The Revised Julian calendar was a variation conceived in 1923 that allowed the years of the Julian calendar (still used by the Eastern Orthodox Church) and Gregorian calendars to remain in step — at least until the year 2799.
Although often overlooked, the extra day in leap years of the Gregorian calendar was originally achieved by repeating the 24th February; a practice inherited from the Julian calendar. It wasn’t until 1662 that it was achieved by adding an extra day at the end of February. Today, a residual repercussion of this is a difference in the date of the feast of St. Matthias during leap years as celebrated by the Catholic and Anglican churches. Earlier still, different Christian churches in Romanised Britain celebrated Easter one day apart. In effect, knowing the name of a celebration or festival doesn’t uniquely determine its date.
The Islamic calendar is the only surviving lunar calendar. It consists of 12 months of alternating 29 and 30 days over a 30-year cycle, except in embolismic years where an intercalary day is added at the end of the 12th month. There are 11 such intercalations and so each 30-year cycle has 10631 days (30 x 12 x 29.5 + 11), and this keeps the month in synchronisation with the moon. There are some variations of the actual years in which the intercalation occurs but it is essentially a rule-based calendar. It is sometimes called the Tabular Islamic calendar in order to distinguish it from the “popular” Islamic calendar where the actual start of each month is determined by a religious authority based on the first visibility of the crescent lunulae of the new moons. This empirical approach is obviously problematic as the appearance depends upon the weather and geographical coordinates, and the date can be one or two days different from the calculated one. It can therefore vary from one part of Islam to another.
India’s calendars are particularly troublesome as they are “…intricate, complex, and subject to numerous local variations”. Most are lunisolar but there are solar calendars too. When India became independent, in 1947, their first Prime Minister, Jawaharlal Nehru, set about a number of reforms, and one of these involved the Indian Calendar Reform Committee, appointed in 1952. They found that there were over 30 well-developed calendars in use across India, and they set about creating a unified Indian national calendar, which was adopted on 22 Mar 1957. However, India’s diverse population meant that the Gregorian calendar was still used (by Christians and for administration), and also the Islamic calendar. Indian and Gregorian dates are therefore presented side-by-side by The Gazette of India, in news broadcasts by All India Radio, and in calendars and communications issued by the Government of India.
In the older Indian calendars, the variations of the lunisolar ones included different month names, the date of the start of the year, the phase of the moon on which the year or month started, intercalation rules, and the era from which years were counted, thus making any attempt at general conversion to Gregorian dates “futile”. However, the solar calendars, used in parts of Bengal and Madras, “were susceptible to almost infinite variations”, thus making it very difficult to convert an Indian date to a Gregorian one without specific knowledge of the calendar and locality.
A similar story of innumerable variations applies to the Chinese calendars.
What I wanted to emphasise in this first post is that conversion from these other calendars to our Gregorian one is not simply a matter of looking at the written form and then doing some arithmetic. Even when the calendar system was rule-based, rather than dependent upon observation or ad hoc decisions, the conversion may need information about the actual calendar variant being used, their location, their religion, etc.
If an historian recorded a length in cubits then a very similar situation would arise since there were different cubit definitions used in different places. A write-up may give an indication of what that length might have been in feet (or metres) but the converted value would need substantiating, and it wouldn’t be a direct replacement for the original information.
To genealogists, a more familiar case might be how we handle dates and ages in our sources. Even if we encounter a date in our own calendar then it may be incomplete, or it may be secondary information. Although many genealogists treat them as “facts”, there’s no such thing in principle. With ages then we should be seeing the issue more clearly: we certainly don’t perform a simple subtraction of the age from the date of recording, and then use that date as a “fact”. Someone may have lied about their age, or it may have been age-next-birthday rather than age-last-birthday (as in early Canadian censuses), or it may have been rounded down (as in 1841 census of England and Wales). The essential point is that the calculated date is not information that was in the consulted source; it is derived from it, and it may need more information in order to make that derivation. Any calendar conversion is also a calculation, and similarly different from source information.
In Part II of this post, I want to look at the representation of dates from other calendars, and why a general scheme should be important to us.
 E. G. Richards, Mapping Time: The Calendar and its History (1998; reprint, Oxford University Press, 2005), pp. 92–97.
 An examination of fossil records has shown that a year once consisted of nearly 400 daily cycles, but finding the length of one such cycle is harder to determine. Observations have shown that the earth’s rotation is slowing down, and calculations suggest that it was once substantially faster — some reports as low as 16 hours per day. That would result in a greater centrifugal force acting against gravity, and things would probably have felt lighter millions of years ago.
 David Ewing Duncan, The Calendar (London: Fourth Estate Ltd, 1998), pp.40–43.
 Richards, p.101.
 Duncan, pp.104–5.
 Richards, p.93, p.234.
 Richards, p.174.
 Richards, p.182.
 Richards, p.177.