Date-JD

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NAME

Date::JD - conversion between flavours of Julian Date

DESCRIPTION

For date and time calculations it is convenient to represent dates by
a simple linear count of days, rather than in a particular calendar.
This is such a good idea that it has been invented several times.
If there were a single such linear count then it would be the obvious
data interchange format between calendar modules.  With several
versions, calendar modules can use such sensible data formats and still
have interoperability problems.  This module tackles that problem,
by performing conversions between different flavours of day count.
These day count systems are generically known as "Julian Dates", after
the most venerable of them.

Among Julian Date systems there are also some non-trivial differences
of concept.  There are systems that count only complete days, and
those that count fractional days also.  There are some that are fixed
to Universal Time (time on the prime meridian), and others that are
interpreted according to a timezone.  Some consider the day to start at

lib/Date/JD.pm  view on Meta::CPAN

    $mjd = jd_to_mjd($jd);
    ($cjdn, $cjdf) = mjd_to_cjdnf($mjd, $tz);
    $rd = cjdn_to_rd($cjdn, $cjdf);

    # and 509 other conversion functions

=head1 DESCRIPTION

For date and time calculations it is convenient to represent dates by
a simple linear count of days, rather than in a particular calendar.
This is such a good idea that it has been invented several times.
If there were a single such linear count then it would be the obvious
data interchange format between calendar modules.  With several
versions, calendar modules can use such sensible data formats and still
have interoperability problems.  This module tackles that problem,
by performing conversions between different flavours of day count.
These day count systems are generically known as "Julian Dates", after
the most venerable of them.

Among Julian Date systems there are also some non-trivial differences
of concept.  There are systems that count only complete days, and
those that count fractional days also.  There are some that are fixed
to Universal Time (time on the prime meridian), and others that are
interpreted according to a timezone.  Some consider the day to start at

lib/Date/JD.pm  view on Meta::CPAN

XYZDN is the integral part of XYZD.  There is also a quantity named
"XYZDF" ("XYZ Day Fraction") which is a count of fractional days since
the XYZDN changed (whether that is noon or midnight).  XYZDF is the
fractional part of XYZD, in the range [0, 1).

This quantity naming pattern is derived from JD (Julian Date) and JDN
(Julian Day Number) which have the described correspondence.  Most of
the other flavours of day count listed below conventionally come in only
one of the two varieties.  The "XYZDF" name type is a neologism.

All calendar dates given are in ISO 8601 form (Gregorian calendar with
astronomical year numbering).  An hour number is appended to each date,
separated by a "T"; hour 00 is midnight at the start of the day and hour
12 is noon in the middle of the day.  An appended "Z" indicates that the
date is to be interpreted in Universal Time (the timezone of the prime
meridian), and so is absolute; where any other timezone is to be used
then this is explicitly noted.

=over

=item JD (Julian Date)

days elapsed since -4713-11-24T12Z.  This epoch is the most recent
coincidence of the first year of the Metonic cycle, indiction cycle, and
day-of-week cycle, using the Julian calendar.  It was correspondingly
named after the Julian calendar, and thus after Julius Caesar.  Some
information can be found at L<http://en.wikipedia.org/wiki/Julian_day>.

=item RJD (Reduced Julian Date)

days elapsed since 1858-11-16T12Z (JD 2400000.0).  Rarely used.

=item MJD (Modified Julian Date)

days elapsed since 1858-11-17T00Z (JD 2400000.5).  This was introduced by
the Smithsonian Astrophysical Observatory in 1957, and is recommended for

lib/Date/JD.pm  view on Meta::CPAN

Confusingly, in the book the integral form is also called "RD".
The integral form is called "RDN" by this module to avoid confusion,
reserving the name "RD" for the fractional form.  (The book is best
treated with caution due to the embarrassingly large number of errors
and instances of muddled thinking.)

=item LD (Lilian Date)

days elapsed since 1582-10-14T00 in the timezone of interest (CJD
2299160.0).  This epoch is the day before the day that the Gregorian
calendar first went into use.  It is named after Aloysius Lilius, the
inventor of the Gregorian calendar.

=back

The interesting differences between these flavours are whether the
day starts at noon or at midnight, and whether they are absolute or
timezone-relative.  Three of the four combinations of these features
exist.  There is no convention for counting days from timezone-relative
noon that the author of this module is aware of.

For more background on these day count systems,