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=head1 NAME

Date::MSD - conversion between flavours of Mars Sol Date

=head1 SYNOPSIS

    use Date::MSD qw(js_to_msd msd_to_cmsdnf cmsdn_to_js);

    $msd = js_to_msd($js);
    ($cmsdn, $cmsdf) = msd_to_cmsdnf($msd, $tz);
    $js = cmsdn_to_js($cmsdn, $cmsdf, $tz);

    # and 69 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 module performs conversions between different flavours of linear
count of Martian solar days ("sols").

Among Martian day count 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 Airy Mean Time (time on the Martian prime meridian), and others that
are interpreted according to a timezone.  The functions of this module
appropriately handle the semantics of all the non-trivial conversions.

The day count systems supported by this module are Mars Sol Date,
Julian Sol, and Chronological Mars Solar Date, each in both integral
and fractional forms.

=head2 Flavours of day count

In the interests of orthogonality, all flavours of day count come in
both integral and fractional varieties.  Generally, there is a quantity
named "XYZ" which is a real count of days since a particular epoch (an
integer plus a fraction) and a corresponding quantity named "XYZN" ("XYZ
Number") which is a count of complete days since the same epoch.  XYZN is
the integral part of XYZ.  There is also a quantity named "XYZF" ("XYZ
Fraction") which is a count of fractional days since the XYZN changed
(at midnight).  XYZF is the fractional part of XYZ, in the range [0, 1).

This quantity naming pattern is derived from the naming of Terran day
counts, particularly JD (Julian Date) and JDN (Julian Day Number) which
have the described correspondence.  The "XYZF" name type is a neologism,
invented for L<Date::JD>.

All calendar dates given are in the Darian calendar for Mars.  An hour
number is appended to each date, separated by a "T"; hour 00 is midnight
at the start of the day.  An appended "Z" indicates that the date is to
be interpreted in the timezone of the prime meridian (Airy Mean Time),
and so is absolute; where any other timezone is to be used then this is
explicitly noted.

=over

=item MSD (Mars Sol Date)

days elapsed since 0140-19-26T00Z (approximately MJD 5521.50
in Terrestrial Time).  This epoch is the most recent near
coincidence of midnight on the Martian prime meridian with noon
on the Terran prime meridian.  MSD is defined by the paper at
L<http://pubs.giss.nasa.gov/docs/2000/2000_Allison_McEwen.pdf>.

=item JS (Julian Sol)

days elapsed since 0000-01-01T00Z (MSD -94129.0) (approximately
MJD -91195.22 in Terrestrial Time).  This epoch is an Airy
midnight approximating the last northward equinox prior to
the first telescopic observations of Mars.  The same epoch is
used for the Darian calendar for Mars.  JS is defined (but not
explicitly) by the document describing the Darian calendar, at
L<http://pweb.jps.net/~tgangale/mars/converter/calendar_clock.htm>.

=item CMSD (Chronological Mars Solar Date)

days elapsed since -0608-23-20T00 in the timezone of interest.
CMSD = MSD + 500000.0 + Zoff, where Zoff is the timezone
offset in fractional days.  CMSD is defined by the memo at
L<http://www.fysh.org/~zefram/time/define_cmsd.txt>.

=back

=head2 Meaning of the day

A day count has meaning only in the context of a particular definition
of "day".  Potentially several time scales could be expressed in terms
of a day count, just as Terran day counts such as MJD are used in the
timescales UT1, UT2, UTC, TAI, TT, TCG, and others.  For a day number
to be meaningful it is necessary to be aware of which kind of day it
is counting.  Conversion between the different time scales is out of
scope for this module.

=cut

package Date::MSD;

{ use 5.006; }
use warnings;
use strict;

use Carp qw(croak);

our $VERSION = "0.005";

use parent "Exporter";
our @EXPORT_OK;

my %msd_flavours = (
	msd => { epoch_msd => 0 },
	js => { epoch_msd => -94129.0 },
	cmsd => { epoch_msd => -500000.0, zone => 1 },
);

=head1 FUNCTIONS

Day counts in this API may be native Perl numbers or C<Math::BigRat>
objects.  Both are acceptable for all parameters, in any combination.
In all conversion functions, the result is of the same type as the
input, provided that the inputs are of consistent type.  If native Perl
numbers are supplied then the conversion is subject to floating point
rounding, and possible overflow if the numbers are extremely large.
The use of C<Math::BigRat> is recommended to avoid these problems.
With C<Math::BigRat> the results are exact.

There are conversion functions between all pairs of day count systems.
This is a total of 72 conversion functions (including 12 identity
functions).

When converting between timezone-relative counts (CMSD) and absolute
counts (MSD, JS), the timezone that is being used must be specified.
It is given in a ZONE argument as a fractional number of days offset
from the base time scale (typically Airy Mean Time).  Beware of