The history of linear time

The dominance of one calendar for world events is quite recent and many other calendars remain in use: the Ethiopian calendar, for ­instance, has 13 months. The references AD and BC are sometimes ­replaced by CE and BCE: Common Era and Before the Common Era.

The Roman calendar was counted Ab urbe condita ("from the foundation of the city"), in 753 BC; and it continued in use until the Anno Dominicalendar was introduced in AD 525. The monk who calculated AD from AUC forgot that the emperor augustus ruled for four years as Octavian before he changed his name, and this error remains in the system. Also, as he counted in Roman, not Arabic, numerals, he did not include the years 0 BC and AD 0.

The Muslim calendar runs from the , Muhammad's flight from Mecca to Medina in AD 622. Like the Christian calendar, it displaced earlier calendars such as the one in Persia, which dates from about 1200 BC. The Muslim calendar is a lunar one, but Iranians still celebrate Nowruz, the new year in the solar ­Zoroastrian calendar, at the spring equinox each March.

The Chinese calendar dates back to about 2700 BC and the Hindu calendar to about 3100 BC. The Jewish calendar has an even earlier starting point, 5,770 years ago, calculated as the date of the creation as described in scripture.
Roger Crosskey, London W10

Official records of the Roman ­empire and its successors used two systems in parallel. One, used in legal documents, dated from the accession of the current emperor, and started again with each new emperor (a system still used with each new ­monarch in English law). The other, used in historical works, was AUC, Ab urbe condita. In "the year of the consulship of Probius Junior" (1278 AUC) , a member of the Roman curia, invented the AD system by recording that it was 525 years "since the incarnation of our Lord Jesus Christ".

He probably arrived at that date by looking up the recorded dates of incidents mentioned in the Christian gospels. Matthew 2:1 has Jesus born during the reign of Herod, who died in 749 AUC after a long illness. Luke 2:2-6 has Jesus born at the time of the census of Judea instituted by Quirinius, which took place in 759 AUC. Dionysius seems to have decided on a compromise, putting the birth of Jesus between the two ascertained dates, at 753 AUC.
Much later, in AD 731 (1484 AUC), the custom of dating events AD, using Dionysius's date, was originated by the historian .