Era
Era · quantity: not a duration but an origin
On 19 December 7208 Peter I signed a decree: after 31 December would come not 1 January 7209 but 1 January 1700. The year itself stayed exactly the same — 365 days and four seasons. Only the number changed. 5508 years were struck from the count at a stroke, and the whole of Russian history moved to a new numbering.
That is what an era is. The unit of time is the same for every people — one turn of the Earth about the Sun. The only difference is which event they started counting from. So one and the same day today is numbered 2026, 7534, 1448 and 2779 — and all four numbers are correct.
A year has a length that can be measured. An era has nothing measurable at all: its beginning is appointed. So an era cannot be checked against an instrument — only against someone else’s record of the same day.
01 · Definition
An era is an event from which the years are counted, and a rule for counting them. Everything else in a calendar is about the length of the year and the months. An era answers one question only: where is the zero.
Events fit to serve as a zero come in four kinds. The beginning of the world — so it was reckoned in Byzantium, among the Jews, in India. The birth or the journey of a founder — the Common Era, the Hijra, the era of the Buddha. The start of a reign — Roman consuls, Japanese nengō, the Meiji era. And an arbitrary point chosen for convenience — the astronomers’ Julian period.
Everything after that rests on a single device. To tie two eras together you place a shared day between them — a moment described in both systems. Astronomers settled this once and for all: they keep an unbroken count of days from 1 January 4713 BC, the Julian date. Convert any era into it, and the calendars begin to speak to one another.
The panel on the right shows the whole point: one axis of time, ten different zeros. The slider picks a day; the bars show how many years each era has counted by then.
The slider picks one and the same day of history. The vertical line is that day. The bars grow leftward from it: each starts at its own zero, and the length of the bar is the year number in that era.
Peter’s decree did two things at once: it moved the start of the year from 1 September to 1 January and it changed the era. The calendar stayed Julian — Russia went over to the New Style only in 1918. So Russian dates of the 18th and 19th centuries carry two separate traps, and they are still confused today.
02 · Conversion
Enter a year and the sheet converts it
The base is the year CE. A negative number means a year BC in the astronomical count, which does have a year zero: astronomical −44 is 45 BC. Historians have no zero, and that is the commonest source of a one-year error.
Lunar eras are reckoned only approximately. The Hijri year is eleven days shorter than the solar one, so the exact number depends on the month. Thirty-three Hijri years make about thirty-two solar ones, and the gap widens steadily.
No era began on 1 January. The Byzantine year opened on 1 September, the Hebrew one at Rosh Hashanah in autumn, the Iranian one at the vernal equinox. This means the shift between two eras is not a single number: for part of the year it is one, for the rest another.
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03 · Orders of magnitude
the depth of an era — from a single reign to a kalpa04 · Measuring instruments
What pins an era to the axis of time
Eclipses can be computed backwards by mechanics thousands of years in either direction. If a chronicle says the sun went dark in such and such a year of a reign, the day can be pinned exactly — and the whole reign with it.
A list of rulers from Nabonassar to the Roman emperors, with the length of every reign in years. Ptolemy drew it up so that his own observations could be dated. It is by this list that Babylonian, Persian and Greek records are still stitched together.
A single sentence such as “in the third year of Cyrus, when so and so was archon at Athens” sews two independent counting systems together. Such remarks in ancient authors are worth more than any chronological treatise.
For ages with neither records nor eclipses, physics does the work: the decay of carbon-14 in a sample, plus calibration against tree rings, which are counted one by one. Rings give an exact year, radiocarbon gives a range — together they give a date.
05 · Writing rules
Name the era when it is not the common one
A year of the Common Era is written plain, a year before it with an abbreviation. English keeps a live quarrel here: AD goes before the number and BC after it, because anno Domini means “in the year of the Lord” — so AD 525 but 44 BC. Neutral texts write CE and BCE, in both cases after the number.
Russian dates need a rule of their own. Until February 1918 Russia used the Julian calendar, so events of the 18th and 19th centuries carry two dates: Old Style and New. The custom is to write both, with the second in brackets. The British have exactly the same trouble: Britain went over to the Gregorian calendar only in 1752, and eleven days disappeared then too.
06 · Neighbouring units
The era stands beside the year and the century but answers for something else. A year says “how many”, a century “how many round hundreds”, an era “counted from where”. The first two are lengths; the third is a point.
a length, measured
a zero, appointed
a hundred, counted
A shift works only for solar eras. Lunar ones count in a different unit: 33 Hijri years are roughly 32 solar ones, so no single constant will convert them — a whole formula is needed.
07 · Historical section
Who moved the zero, and when
The monk Dionysius Exiguus was drawing up tables of Easter and did not wish to go on counting from Diocletian, the emperor who had executed Christians. He reckoned the year of the Nativity and began from it. He was out by some four to six years — but the count took hold and became the world’s.
The count from the Hijra was introduced seventeen years after the event itself, under the caliph Umar. The zero taken was neither the birth of Muhammad nor his death but the migration of the community to Medina: not a person but a beginning of a community. The year is lunar, 354 days, and so it drifts against the seasons.
Rus counted the years by the Byzantine era from the creation of the world. Peter moved the country to counting from the Nativity and shifted the start of the year from September to January. Church books kept the old count without a break, which is why the two numbers can still be seen side by side.
The Revolution appointed the abolition of the monarchy as its zero and rewrote everything: twelve months of thirty days, décades instead of weeks, new month names. It lasted thirteen years. The era fell not because the arithmetic failed but because the week proved stronger than the decree.
Where 7534 actually comes from
The number from the “Slavic calendar” is no invention. It is the genuine year of the Byzantine era: 2026 plus 5508. That is exactly how the years were counted in Rus before Peter, and church books count them so to this day.
What is disputed is not the figure but the story pinned to it in the late twentieth century: that the zero of the era marks some battle, a “creation of the world in the Star Temple”. Byzantium computed that date from the Bible in the sixth century, and there is no battle anywhere in the sources.
Here is what is interesting about all this from the point of view of metrology. An era can be neither confirmed nor refuted by measurement — it has no standard. It can only be checked for consistency: does it agree with other counts on a shared day. This is the one quantity on these pages where the question “is it true?” has no meaning at all.
Catalogue · units of measurement
A passport for every quantity
Seven SI base units, twenty-two derived ones with names of their own, and the non-SI quantities that neither science nor daily life can do without. Each gets its own sheet: definition, conversion, instruments, writing rules, history. In 36 languages.