[image of logo] Julian and Modified Julian Day Routines

The Julian Day system counts sequential days since come time in the past. By decree, Julian Day (JD) zero is noon on 1 January in the year -4712 which is 4713 BC. This running day counter should not be confused with Day of Year (DOY) which is commonly, but incorrectly, termed Julian Day.

Modified Julian Day (MJD) was defined in the mid 1950's in the interests of astronomy and space science as MJD = JD - 2400000.5, which accomplishes two things. The half day shift makes the day start at midnight, which is the current time standard. Subtracting the large number shifts the zero day to a more recent time (May 23, 1968) allowing smaller numbers to represent time.

Matlab and FORTRAN routines are provided for conversion between Gregorian dates and Modified Julian dates.

For more information on these issues, please see Numberical Recipes, or the Navy Official Time site history page (tycho.usno.navy.mil/history.html).

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Written by J. Klinck.

July, 2000.

Send questions or comments by e-mail to klinck@ccpo.odu.edu