Modified Julian Dates

The time records in these Pathfinder products use Coordinated Universal Time (UTC), expressed in terms of Modified Julian Dates. This page describes what is meant by Modified Julian Dates and also gives pointers that help with conversions to month/day/year.

MJD is a modification of the Julian Date that is routinely used by astronomers, geodesists, and even some historians. This dating convention, designed to facilitate chronological calculations, numbers all days in consecutive fashion, beginning at a date sufficiently far into the past so as to precede the historical period. Julian Day Number is an integer counter of the days beginning at noon 1 January 4713 B.C., which is Julian Day Number 0. The Julian Date is simply the extension of the Day Number to include a real fraction of day, allowing a continuous time unit. The use of noon as the beginnning of a day is also an astronomers' convention; it allows their usual nighttime observing session to maintain a single date without break.

MJD modifies this Julian Date in two ways. The MJD begins at midnight rather than noon, in keeping with more standard conventions. Secondly, for simplicity, the first two digits of the Julian Date are removed. This is because, for some three centuries following 17 November 1858, the Julian day lies between 2400000 and 2500000. The MJD drops those first `24' digits. Thus, we have

MJD = JD - 2400000.5

To help users convert our MJD times in the Pathfinder data to calendar dates, we make available a subroutine CALDAT which is a slightly modified version of the code given in Numerical Recipes by Press, Flannery, Teulosky, and Vettering. To use it, let M be the integer part of the MJD. Then the user calls the subroutine with a Julian day J, where

J = M + 2400001 The routine then returns the month, day, and year.

As an additional aid, we also give tables allowing calendar days and MJD to be easily converted for any day in the decade of the 1980s, 1990s, and 2000s. To use these tables, add the day-of-month to the tabulated entry.

Click here for the table for the 1980s.

Click here for the table for the 1990s.

Click here for the table for the 2000s.

Page maintained by R. Ray.

Last modified 25/4/97.