Date and time systems

Dates

In text, spell out the names of the days and months in full, and use the sequence day–month–year:

The sequence year–month–day is used in some countries, including the United States, so spelling out the month rather than using a numeric value (e.g. 5 for May) is important to avoid ambiguity.

Abbreviated forms may be used in tables and graphs; use the first 3 letters and no full stop:

Year spans are expressed as:

• a range of years using an en dash (not a slash); truncate the second year to 2 digits unless spanning a century

• a reference to a decade

Apostrophes are not needed in plural expressions of time:

but are required in the singular:

Clock time

In Australia, time in written text usually uses the 12-hour clock:

To avoid confusion, do not use am or pm when referring to noon and midnight:

Spell out all units of time, and use numerals rather than words for numbers:

The rule to spell out units of time can be relaxed in tables if it is critical to save space, but avoid confusion with other units:

The abbreviation for year can be either y or yr; maintain consistency within the publication.

For parts of hours, minutes and seconds, close up the values:

or use decimal fractions:

Like other units, abbreviations of units of time are not written with a full stop, nor do they take a plural s when referring to numbers greater than 1.

Time systems

International atomic time (TAI) is a timescale based on the average of about 400 atomic clocks kept throughout the world. It is displayed in the same way as coordinated universal time (UTC):

UTC is the time-keeping method familiar to, and used by, civilians – it is Greenwich mean time (GMT) counted from midnight, while GMT is taken from noon. UTC is approximately 35 seconds behind TAI:

Terrestrial time (TT) is TAI + 32.184 seconds.

Greenwich mean sidereal time (GMST) is based on angles – that is, relative to Earth’s rotation and orbit.

Universal time (UT) is based on Earth’s rotation with respect to the Sun and, like GMST, refers to angles rather than an actual timescale.

TT, GMT and UT are displayed in the same way as other time systems.

Australian time zones

In 2005, UTC was adopted as the standard for all Australian standard times. Three formal time zones cover the main island of Australia: AEST (Australian eastern standard time), ACST (Australian central standard time) and AWST (Australian western standard time). Australia’s external territories (including the Australian Antarctic Territory) occur in other time zones.

Express times including the time zone as:

Preface the time zone with A. Use initial capitals for the abbreviation; when writing the term out in full, use lower case except for proper nouns.

In states where daylight saving time (DST) is observed, clocks are advanced 1 hour on the first Sunday in October. Either Australian eastern daylight time (AEDT) or Australian central daylight time (ACDT) applies. The Northern Territory, Queensland and Western Australia do not observe DST. DST ends on the first Sunday in April.

Central western standard time, an unoffical time (UTC+8:45, without DST), is used in 1 area in the southeastern corner of Western Australia (towns east of Caiguna on the Eyre Highway, including Eucla, Madura and Mundrabilla) and Border Village in South Australia.

Australian time zones

ACDT = Australian central daylight time; ACST = Australian central standard time; AEDT = Australian eastern daylight time; AEST = Australian eastern standard time; AWST = Australian western standard time; LHDT = Lord Howe daylight time; LHST = Lord Howe standard time

a  Australia's external territories have their own time zones.

The International Date Line crossing the Pacific Ocean changes the date by 1 day. If you cross the date line moving east, you lose a day; if travelling west, you gain a day (with local variations).

Calendars

Capitalise only the main word:

Julian calendar     Gregorian calendar

Historical time

The designations BC (before Christ) and AD (anno domini, ‘in the year of our Lord’, meaning after the birth of Christ) have been largely replaced by BCE (before the common era) and CE (of the common era) to avoid the strongly Christian connotations of the older terms. Set any of these abbreviations without stops, and with a space between the year and the letters:

Note: AD was formerly placed before the year, but this has recently changed to after the year.

In references to centuries, the shortened form for the era follows the century:

Centuries are expressed in the following format:

Before present

Before present (BP) years is a timescale used mainly in geology, archaeology and other scientific disciplines to specify when events in the past occurred. It is usually used when dates have been established by radiocarbon dating, using 1 January 1950 as the starting date of the age scale. This scale is also sometimes called ‘before physics’, which means before nuclear weapons testing altered the proportion of carbon isotopes in the atmosphere, making dating after that time much less reliable:

Julian dates and besselian years

The julian date (JD) is a continuous measurement of days since noon universal time (UT) on 1 January 4713 BC. Astronomical events are often represented by the JD, followed by a conventional date in UT:

If the date needs to be displayed as numerals, use hyphens:

There are many online tools that will convert JD to UT and vice versa (e.g. the American Association of Variable Star Observers julian date converter(Opens in a new tab/window)).

A julian century is 36,525 julian days. Julian epochs are denoted by a J:

Besselian epochs (denoted by a B) were used before 1984; modern publications should use julian epochs:

Historical ages

Use initial capitals for the names of ages used to describe prehistory:

Also use initial capitals for the names of cultural periods:

Use initial capitals for the names of ice ages, but use lower case for generic terms:

For prehistoric periods (e.g. Jurassic, Ediacaran) see History of Earth.