The Julian Date (JD) is the interval of time in days and fractions of a day since noon on 1 January 4713 BC Greenwich Mean Time (GMT) in the Julian proleptic calendar (see below).

The Julian Day Number (JDN)

The Julian Day Number (JDN) is the integer value of the Julian date (JD). It is often called the Julian Day (JD), although this is not technically correct. The day commencing at noon 1 January 4713 BC GMT is JDN 0. Negative values can be used for preceding dates, though they predate all recorded history.

Astronomical Julian Day (AJD)

Although usage of terms relating to Julian Dates does vary, the term Astronomical Julian Day (AJD) is often used to denote the Julian Day Number (JDN). This is the term we tend to use in contrast to Chronological Julian Day (CJD).

Chronological Julian Day (CJD)

This is the most well known version of the Julian Day. It is a modification of the Astronomical Julian Day in which the starting point is set at midnight 1 January 4713 BC (Julian calendar) rather than noon.

Chronologists/chronographers found the Julian Day concept useful, but they didn't like noon as starting time because many of our calendars start their days at midnight. So CJD = AJD + 0.5.

There is one other difference between CJD and AJD. AJD uses Coordinated Universal Time (UTC), and so it is the same for all time zones and is independent of Daylight-Saving Times (DST). On the other hand, CJD is not, so it changes with different time zones and takes into account the different local DSTs.

Bible Chronology uses CJD GMT

For consistency and ease of understanding, Bible Chronology uses CJD Greenwich Mean Time. This means it doesn't allow for different time zones and DSTs.

Proleptic Calendar

A calendar that is obtained by projection backwards earlier in time than its invention or implementation is called the "proleptic" version of the calendar. The Julian calendar was started in 45 BC but the leap years actually observed between its official implementation and AD 4 were erratic and so the Julian calendar projected back before AD 4 is the Julian proleptic calendar. It uses the calendar's rules that were applied consistently after 4 AD.

Likewise, the proleptic Gregorian calendar is occasionally used to specify dates before its official introduction in 1582. Before 1582, it should properly be called the Gregorian proleptic calendar.

AD/BC

See separate article on The Anno Domini Dating System