Julian date

The Julian date (Julian day, JD) is a notation that specifies a date and time with a single decimal number, the integer-portion specifying the number of noon-to-noon days since noon, November 24, 4714 BC (Gregorian calendar), and the fractional part specifying the additional fraction of a 24-hour day. For example, Julian date 2451545.0 represents noon January 1, 2000 and 2451544.958333 (i.e., 2451545.0 minus 1/24) represents 11AM of the same day. Julian date notation can be used with your choice of time standards; the IAU recommends Terrestrial Time (TT) as the default for astronomical purposes. Note that depending upon the time standard used, it can be necessary to adjust calculated time-intervals to accommodate leap seconds. Some Julian date variants:

• modified Julian day (MJD) is JD minus 2400000.5, for convenience in writing a smaller number.
• heliocentric Julian day (HJD) is JD corrected to the position of the Sun. In astronomy, when placing the time of an observation, it is the time that the observation would reach the Sun.
• barycentric Julian day (BJD) is similar but for the center of mass of the solar system. It varies from HJD by up to 4 seconds. It has become standard for astronomical observations.

MHJD can mean "modified HJD" as above, or can be used for a version adjusted by some other constant. Analogously for MBJD.

The term Julian year is somewhat distinct from Julian dates and the Julian calendar: it is a unit of time (e.g., for citing the length of a time interval) used in astronomy that is exactly 365.25 days of 86400 SI seconds.

Be warned that the terms Julian day and Julian date are often (outside time standards and astronomy) used with other meanings, particularly, a count of days from the beginning of the current year. Such usage varies: the count may start at either 0 or 1 for January 1, and sometimes an indication of the year is included, such as appending the three-digit day count to the last two digits of the year, e.g., 22032 for February 1, 2022.