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For a general discussion of Julian day numbers please see Julian Day Numbers, and in particular the explanation of the difference between astronomical and chronological Julian day numbers given in Section 4 and in Section 5.
To put it briefly, astronomical Julian days correspond to 24-hour periods which begin at noon GMT, with 0 AJD beginning at noon on -4712-01-01 in the Julian Calendar (JC), whereas chronological Julian days (relative to the Greenwich meridian of zero degrees longitude) correspond to 24-hour periods which begin at midnight GMT at the start of -4712-01-01 JC. A Julian date is a Julian day number with a fractional component representing the time elapsed since the start of the day (or rather the nychthemeron, the day-and-night). Thus the astronomical Julian date 1.25 AJD corresponds to 18:00 GMT on -4712-01-02 JC, whereas the chronological Julian date 1.25 CJD corresponds to 06:00 GMT on -4712-01-02 JC. Chronological Julian dates relative to the Greenwich meridian (zero degrees longitude) are always 0.5 greater than the corresponding astronomical Julian dates (since 0.5 days = 12 hours).
An astronomical Julian date is independent of location on the Earth, since it is always relative to the prime meridian of zero degrees. A chronological Julian date, however, is longitude-dependent. A chronological Julian day begins at midnight local time, not midnight GMT. (However, when the concept is used without reference to any particular location then by default the prime meridian of zero degrees is assumed.)
Checking 'Show chronological JDs' will cause two chronological Julian date values to be displayed, namely, those for China (120° East) and London, e.g.:
The chronological Julian date in London equals the astronomical Julian date (AJD) + 1/2. The chronological Julian date for China (CJDC) depends on the longitude of the prime meridian used for the Chinese Calendar. Normally 120° East is used, in which case CJDC equals AJD + 1/2 + 1/3 (since China time is 8 hours ahead of GMT and 8/24 = 1/3).
You can enter a chronological Julian date in the 'Date' box either by (a) clicking on the date displayed or (b) entering a number such as 2,322,110 together with one of 'CJDC' (China) and 'CJDL' (London). The value entered can include commas and a fractional component, e.g., 2,421,220.3991.
Chronological Julian dates refer to particular instants, since they can have any real value. In contrast, Julian day numbers have integer values, and are used to denote days (at a particular location). See also Message Concerning Chronological Julian Days/Dates.
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