⏾ Concept Map

Modified Julian Day (MJD) ○◂ Entries

↩ 🔵 🔵 🔵

Primary

၊၊||၊|။

Modified Julian Day (MJD) ○◂｜Definition｜1st｜20251119205401-00-⌔

Julian day - Wikipedia#Variants

Variants

Because the starting point or reference epoch is so long ago, numbers in the Julian day can be quite large and cumbersome. A more recent starting point is sometimes used, for instance by dropping the leading digits, in order to fit into limited computer memory with an adequate amount of precision. In the following table, times are given in 24-hour notation.

In the table below, Epoch refers to the point in time used to set the origin (usually zero, but (1) where explicitly indicated) of the alternative convention being discussed in that row. The date given is a Gregorian calendar date unless otherwise specified. JD stands for Julian Date. 0h is 00:00 midnight, 12h is 12:00 noon, UT unless otherwise specified. Current value is at 18:47, Saturday, June 27, 2026 (UTC) and may be cached. []

📊 ➺

  • The Modified Julian Date (MJD) was introduced by the Smithsonian Astrophysical Observatory in 1957 to record the orbit of Sputnik via an IBM 704 (36-bit machine) and using only 18 bits until August 7, 2576. MJD is the epoch of VAX/VMS and its successor OpenVMS, using 63-bit date/time, which allows times to be stored up to July 31, 31086, 02:48:05.47.1 The MJD has a starting point of midnight on November 17, 1858, and is computed by MJD = JD − 2400000.52
  • The Truncated Julian Day (TJD) was introduced by NASA/Goddard in 1979 as part of a parallel grouped binary time code (PB-5) “designed specifically, although not exclusively, for spacecraft applications”. TJD was a 4-digit day count from MJD 40000, which was May 24, 1968, represented as a 14-bit binary number. Since this code was limited to four digits, TJD recycled to zero on MJD 50000, or October 10, 1995, “which gives a long ambiguity period of 27.4 years”. (NASA codes PB-1–PB-4 used a 3-digit day-of-year count.) Only whole days are represented. Time of day is expressed by a count of seconds of a day, plus optional milliseconds, microseconds and nanoseconds in separate fields. Later PB-5J was introduced which increased the TJD field to 16 bits, allowing values up to 65535, which will occur in the year 2147. There are five digits recorded after TJD 9999.34
  • The Dublin Julian Date (DJD) is the number of days that has elapsed since the epoch of the solar and lunar ephemerides used from 1900 through 1983, Newcomb’s Tables of the Sun and Ernest W. Brown’s Tables of the Motion of the Moon (1919). This epoch was noon UT on January 0, 1900, which is the same as noon UT on December 31, 1899. The DJD was defined by the International Astronomical Union at their meeting in Dublin, Ireland, in 1955.5
  • The Lilian day number is a count of days of the Gregorian calendar and not defined relative to the Julian Date. It is an integer applied to a whole day; day 1 was October 15, 1582, which was the day the Gregorian calendar went into effect. The original paper defining it makes no mention of the time zone, and no mention of time-of-day.6 It was named for Aloysius Lilius, the principal author of the Gregorian calendar.7
  • Rata Die is a system used in Rexx, Go and Python.8 Some implementations or options use Universal Time, others use local time. Day 1 is January 1, 1, that is, the first day of the Christian or Common Era in the proleptic Gregorian calendar.9 In Rexx, January 1 is Day 0.10
  • The Heliocentric Julian Day (HJD) is the same as the Julian day, but adjusted to the frame of reference of the Sun, and thus can differ from the Julian day by as much as 8.3 minutes (498 seconds), that being the time it takes light to reach Earth from the Sun.11

Printed 2026-06-28.

(echo:: @ ᯤ)

Footnotes

  1. “38 Why Is Wednesday November 17, 1858 The Base Time For VAX/VMS?”. Digital Equipment Corporation-Customer Support Center. Colorado Springs. June 6, 2007. Archived from the original on June 6, 2007. ↩

  2. Winkler n.d. ↩

  3. Chi 1979. ↩

  4. SPD Toolkit Time Notes 2014. ↩

  5. Ransom c. 1988 ↩

  6. Ohms 1986 ↩

  7. IBM 2004. ↩

  8. “datetime – Basic date and time types – date Objects” (December 5, 2021). The Python Standard Library. ↩

  9. Dershowitz & Reingold 2008, 10, 351, 353, Appendix B. ↩

  10. “Chapter 3. Functions – DATE – Base” (September 29, 2022). z/VM: 7.1 REXX/VM Reference ↩

  11. To illustrate the ambiguity that could arise from conflating Heliocentric time and Terrestrial time, consider the two separate astronomical measurements of an astronomical object from the Earth: Assume that three objects – the Earth, the Sun, and the astronomical object targeted, that is whose distance is to be measured – happen to be in a straight line for both measures. However, for the first measurement, the Earth is between the Sun and the targeted object, and for the second, the Earth is on the opposite side of the Sun from that object. Then, the two measurements would differ by about 1000 light-seconds: For the first measurement, the Earth is roughly 500 light seconds closer to the target than the Sun, and roughly 500 light seconds further from the target astronomical object than the Sun for the second measure. An error of about 1000 light-seconds is over 1% of a light-day, which can be a significant error when measuring temporal phenomena for short period astronomical objects over long time intervals. To clarify this issue, the ordinary Julian day is sometimes referred to as the Geocentric Julian Day (GJD) in order to distinguish it from HJD. ↩

Link to original

Secondary

• • •