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Provides a DateTime::Julian class (a subclass of Raku's class DateTime) that is instantiated by either a Julian Date (JD) or a Modified Julian Date (MJD) (or any of the DateTime instantiation methods).

It also provides addional time data not provided by the core class.


use DateTime::Julian;
my $jd  = nnnn.nnnn; # Julian Date for some event
my $mjd = nnnn.nnnn; # Modified Julian Date for some event
my $utc  = DateTime::Julian.new: :julian-date($jd);
my $utc2 = DateTime::Julian.new: :modified-julian-date($mjd);
my $d = DateTime::Julian.new(now);


Module DateTime::Julian defines a class (inherited from a Raku DateTime class) that is instantiated from a Julian Date or a Modified Julian Date.

Following are some pertinent definitions from Wikipedia topic Julian day:

The following methods and routines were developed from the descriptions of code in References 1 and 2. The author of Ref. 3 has been very helpful with this author's questions about astronomy and the implementation of astronomical routines.

The main purpose of this module is to simplify time and handling for this author who still finds Julian dates to be somewhat mysterious, but absolutely necessary for dealing with astronomy and predicting object positions, especially the Sun and Moon, for local observation and producing astronomical almanacs.

This module will play a major supporting role in this author's planned Raku module Astro::Almanac;

Class DateTime::Julian methods

method new

new(:$julian-date, :$modified-julian-date) {...}

If both arguments are entered, the Julian Date is used. If neither is entered, the user is expected to use one of the normal DateTime creation methods.

method jdcent2000

jdcent2000(--> Real:D) {...}
# alternatively use aliases:
method cent2000(--> Real:D)  {...}
method c2000(--> Real:D)     {...}
method jdc2000(--> Real:D)   {...}
method t2000(--> Real:D)     {...}
method jc2000(--> Real:D)    {...}

Returns time as the number of Julian centuries since epoch J2000.0 (time value used by Astro::Montenbruck for planet position calculations).

Exported constants

Several commonly used astronautical constants are exported.


Returns the Julian Date value for the Modified Julian Date epoch of 1858-11-17T00:00:00Z.

say MJD0;               # OUTPUT: «2400000.5␤»
# alternatively use aliases:
say mjd0;               # OUTPUT: «2400000.5␤»


Returns the Julian Date value for the POSIX (Unix) epoch of 1970-01-01T00:00:00Z.

say POSIX0;               # OUTPUT: «2440587.5␤»
# alternatively use alias:
say posix0;               # OUTPUT: «2440587.5␤»


The last day the Julian calendar was used. A DateTime object for 1582-10-04T00:00:00Z.

say JCE;                 # OUTPUT: «1582-10-04T00:00:00Z␤»
# alternatively use alias:
say jce;                 # OUTPUT: «1582-10-04T00:00:00Z␤»


The official start date for the Gregorian calendar. A DateTime object for 1582-1014T00:00:00Z. The days of 5-14 were skipped (the 10 "lost days").

say GC0;                 # OUTPUT: «1582-10-15T00:00:00Z␤»
# alternatively use alias:
say gc0;                 # OUTPUT: «1582-10-15T00:00:00Z␤»


Julian date for 2000-01-01T12:00:00Z (astronomical epoch 2000.0).

say J2000;               # OUTPUT: «2451545␤»
# alternatively use alias:
say j2000;               # OUTPUT: «2451545␤»


Julian date for 1899-12-31T12:00:00Z (astronomical epoch 1900.0).

say J1900;               # OUTPUT: «2415020␤»
# alternatively use alias:
say j1900;               # OUTPUT: «2415020␤»


Seconds per 24-hour day.

say sec-per-day;         # OUTPUT: «86400␤»


Seconds per Julian century.

say sec-per-jcen;        # OUTPUT: «3155760000␤»


Days per Julian century.

say days-per-jcen;       # OUTPUT: «36525␤»


Difference between Sidereal and Solar hour (the former is shorter).

say solar2sidereal;      # OUTPUT: «1.002737909350795␤»

To do

Add calculations (and tests) for Ephemeral Time and Universal Time. (See Ref. 1, section 3.4, p. 41.)


  1. A proleptic calendar, according to Wikipedia, "is a calendar that is applied to dates before its introduction."


  1. Astronomy on the Personal Computer, Oliver Montenbruck and Thomas Pfleger, Springer, 2000.

  2. Celestial Calculations: A Gentle Introduction to Computational Astronomy, J. L. Lawrence, The MIT Press, 2019.

  3. Astro::Montenbruck, Sergey Krushinsky, CPAN.

  4. Mapping Time: The Calendar and Its History, E. G. Richards, Oxford University Press, 2000.

  5. Date Algorithms (Version 5), Peter Baum, Aesir Research, 2020, https://researchgate.net/publication/316558298.


Tom Browder (tbrowder@acm.org)


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