The natural units of time are the solar day, the lunar month and the solar year. Most ancient people ended one day, and so started the next, at sunset, and started the month on the first evening the New Moon was visible. They usually started the year at or about the winter solstice or vernal equinox. So dates within a year were easy, for example the 3rd day of the 11th month. But there is no natural point to start counting years from: any year numbering system is Man-made and arbitrary.
Many ancient civilisations numbered years from events such as astronomical or meteorological phenomena or the death or accession of a King or Emperor, for example “the year after the comet”, or “three years after the whirlwind”, or “in the fifteenth year of the reign of Tiberius Caesar”. In 525 a monk called Dionysius Exiguus (Denis the Short) was making Year Tables for the date of Easter Sunday and decided to use a year numbering system starting from the year of our Lord’s Incarnation (that is, the birth of Jesus) rather than a date associated with a pagan Roman Emperor. The years after our Lord’s Incarnation he referred to as Anno Domini (“in the year of Our Lord”, or A.D.). It was however another two hundred years before his system was widely used throughout the Christian world. The Jarrow monk Bede (673 - 735) was the first writer to use it, in his Ecclesiastical History of the English People, completed in 731 (and written in Latin of course). It was only much later, after Latin had fallen out of general use, that B.C. (“before Christ”) began to be used. Note that we write and refer to A.D. 1066 but 480 B.C..
There is no A.D. 0 - for an
explanation of this please see Note.
We now believe that Dionysius made a mistake in his calculations and that Jesus was probably born about 4 B.C. or possibly 7 B.C. or a little earlier, but the numbering system he introduced has not been changed.
Jews, Christians and Moslems, and most other religions, have their own calendars and year numbering systems. In the 1950s a group of Jewish, Islamic and Christian scholars were working together in Jerusalem on those writings which are sacred to all three faiths, which Christians call the Old Testament. They all felt that their work would be a lot easier if they used a common system for dating documents and events. So in a most wonderful example of inter-faith co-operation they introduced the system now known as the Common Era, which uses as its starting point the (traditional) date of the birth of Jesus, a person who all three faiths accept as real and good. They chose this event because it led to a system similar to one with which they were all already familiar, whether or not they had used it themselves.
It is necessary and quite inoffensive to measure elapsed time forwards and backwards from a specific historical event, and the birth of Jesus is one such event although any other would do. The problem for many people, including Jews and Moslems, is not dating an era from the historical event of the birth of Jesus but the attributes given to the person of Jesus by the use of the titles Christ and Our Lord which are incorporated into the terms A.D. and B.C. and are deeply offensive to many people, including of course Jews and Moslems. Common Era has no connotations of any sort.
It is important to understand that the Common Era was introduced only so that Jewish, Islamic and Christian scholars and historians could have a common system of dating documents and events common to all three: it was never intended that it should replace all other calendars and year numbering systems. Jews, Moslems, Hindus, Sikhs, Buddhists, and of course Christians, can all use it without offence, without in any way ceasing to use their own calendars and year numbering systems. Few Christians can be offended by a system which dates things from the birth of Jesus rather than the birth of Christ. What is offensive to many people, including Christians, is the way in which the politically correct lobby have hi-jacked it, so that the use of any other system, but particularly A.D./B.C., is considered politically incorrect.
As said above the starting point for any year numbering system is Man-made and arbitrary: if we say that we are taking as our starting point the birth of Jesus and count everything from that then we can say that the Battle of Hastings is 1066 CE. If we then find out that the birth of Jesus was not in 1 CE, or even if we find out that Jesus did not live at all, this does not matter: we just redefine our datum to be 1066 years before the Battle of Hastings. This is what we are doing all the time in other fields: the second was once defined as a certain fraction of a day, then as a certain fraction of the tropical year 1900, and now it is defined as the time for a certain number of vibrations of a caesium atom. This is further discussed elsewhere in this Paper, in The Earth as a Timekeeper
Astronomers use a Julian Date, the origin for that being set far enough back to ensure that all historically recorded astronomical events have taken place since then (and the Julian Day starts at noon so that all events for one night occur on the same Julian date). Julian dates are also often used by historians and other people - see the last Paragraph of this Page. For more about the Julian date please see the Section on the Julian Calendar Page.
At the end of the 18th century the French Revolutionary Government tried to introduce a metric day, week, month and calendar and a new origin for the year numbering system, mainly to get away from what they perceived to be the religious connotations of the old ones, but once he became Emperor Napoleon would have no truck with their ideas. The French Revolutionary Calendar is discussed in greater detail on another web site which you can link to from the Gregorian Calendar page.
Historians usually omit not only the CE but also the BCE, as in most cases the context does not permit any confusion, for example the Battle of Salamis 480 and the Battle of Hastings 1066. But first century dates (CE or BCE) are usually signed, for example +70 for 70 CE and -44 for 44 BCE. They also usually sign CE dates in an article in which most of the dates are BCE, and vice versa.
Remember that the year before +1 is -1 not 0. People who want to do date calculations spanning this time, and also the change from the Julian to Gregorian Calendar, often use Julian dates.
Geologists deal in very much longer time scales and usually give dates in a form such as 60 mya (sixty million years ago)!