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:
450 BCE 2015 CE 80 BC 200 AD
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:
in the first century CE the second century BCE
Centuries are expressed in the following format:
21st century 15th century
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:
a date of 7500 years BP
the radiocarbon date (700 BP)
changed from ca 2000 BP to ca 7000 BP
1000 cal BP [indicating that the date has been calibrated to tree ring dates]
Julian dates and besselian years
The julian date (JD) is a continuous measurement of days since noon universal time (UT) (see Time systems) on 1 January 4713 BC. Astronomical events are often represented by the JD, followed by a conventional date in UT:
JD 2456735.70883 = 19 March 2014, 05:03 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 (eg https://www.aavso.org/jd-calculator).
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:
