11 Numbers and dates

11.6 Decades, centuries, and eras

11.6.1 Decades

References to decades may be made in either words or figures:

the sixties

in his seventies

the 1960s

in his 70s

Write either the sixties or the 1960s, not the ’60s. Similarly, when referring to two decades use the 1970s and 1980s, even though the 1970s and ’80s transcribes how such dates may be read out loud.

When the name of a decade is used to define a social or cultural period it should be written as a word (some styles use an initial capital). The difference between labelling a decade the twenties and calling it the 1920s is that the word form connotes all the social, cultural, and political conditions unique to or significant in that decade, while the numerical form is simply the label for the time span. So, the frivolous, fun-loving flappers of the twenties, but the oyster blight of the 1920s.

11.6.2 Centuries

Depending on the editorial style of the work, refer to centuries in words or figures; Oxford style is to use words:

the nineteenth century

the first century bc

Centuries may be abbreviated in notes, references, and tabular matter; the abbreviation may be either c. or cent.: 14th c., 21st cent. Both spelled-out and abbreviated forms require a hyphen when used adjectivally:

an eighth-century (or 8th-c.) poem
the early seventeenth-century (or 17th-c.) dramatists

In dating medieval manuscripts, the abbreviation s. (for saeculum, pl. ss.) is often used instead: s. viii.

Centuries bc run backwards, so that the fifth century bc spanned 500–401 bc. The year 280 bc was in the third century bc.

Conventions for numbering centuries in other languages vary, though for the most part capital Roman numerals are used, with two common exceptions: French uses small capitals in roman, full capitals in italic. In either case the figures are followed by a superior e or ème to indicate the suffix: le XVIIe siècle, le XVIIème siècle. German uses Arabic numerals followed by a full point: das 18. Jahrhundert. Occasionally capital Roman numerals are used; these too must be followed by a full point.

11.6.3 Eras

The two abbreviations most commonly used for eras are bc and ad. Both are written in small capitals. The abbreviation bc (before Christ) is placed after the numerals, as in 72 bc, not bc 72; ad (anno domini, ‘in the year of our Lord’) should be placed before the numerals, as in ad 375 (not 375 ad). However, when the date is spelled out it is normal to write the third century ad rather than ad the third century.

Some writers prefer to use ad for any date before the first millennium. While this is not strictly necessary, it can be handy as a clarifying label: This was true from 37 may not instantly be recognized as referring to a year. Any contentious date, or any year span ranging on either side of the birth of Christ, should be clarified by bc or ad: this was true from 43 bc to ad 18. Conversely, a date span wholly in bc or ad technically needs no clarification, since 407–346 is manifestly different from 346–409, though it is customary to identify all bc dates explicitly.

The following eras should be indicated by the appropriate abbreviation before the year:

  • a.Abr. (the year of Abraham), reckoned from 2016 bc and used in chronicles by Eusebius and Jerome; not written aa.

  • ah (anno Hegirae, ‘in the year of the Hegira’), the Muslim era, reckoned from 16 July 622 (the date of Muhammad’s departure from Mecca).

  • am (anno mundi, ‘in the year of the world’) will normally represent the Jewish era, reckoned from 7 October 3761 bc.

  • as (anno Seleuci), the Seleucid era, variously reckoned from autumn 312 bc and spring 311 bc, formerly current in much of the Near East.

  • auc (anno Urbis conditae, ‘in the year of the foundation of the City’), the supposed Roman era from 753 bc. This was actually used only rarely by the Romans (who had several different dates for the foundation of Rome, and designated the year by the names of the consuls).

The following eras should be indicated by the appropriate abbreviation after the year:

  • bce (Before Common Era) and ce (Common Era) are used instead of bc and ad, mainly by writers who wish to avoid specifying dates in Christian terms.

  • bp (Before Present) is used by geologists and palaeontologists for dates not accurate within a few thousand years; ad 1950 is fixed as the conventional ‘present’. It is customary to use bp when discussing periods before 10,000 years ago. Some authors favour bp as a matter of course, since it does not presuppose any Christian reckoning on the reader’s part. This is acceptable, provided bp is not intermingled with bc and other references of this kind within the same text.

For all era abbreviations other than a.Abr., use unspaced small capitals, even in italic.


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