#!/usr/bin/perl -w

use strict;
use Time::Local;
use constant PERIOD => 29.530588853;
use constant OFFSET => 0;


sub moon_age($)
{
   my ($t) = @_;

   my ($sec, $min, $hour, $mday, $mon, $year, $wday, $yday) = gmtime($t);

   my $days = $year * 365
              + int(($year - 1) / 4)
              - int(($year - 1) / 100)
              + int(($year - 1 + 1900) / 400) - 4
              + $yday;

   $days += $hour / 24.0 + $min / (24.0 * 60.0) + $sec / (24.0 * 60.0 * 60.0);

   my $phase = $days + OFFSET;
   $phase -= int($phase / PERIOD) * PERIOD;
   printf '%04d-%02d-%02d %02d:%02d:%02d  days = %d, phase = %.10f'."\n",
          $year + 1900, $mon + 1, $mday, $hour, $min, $sec, $days, $phase;
}

moon_age(timelocal(0,  0,  0,   1,  1 - 1, 1900));
moon_age(timelocal(0,  0,  0,  11,  8 - 1, 1999 - 1900));
moon_age(timelocal(0, 16, 11,  28,  1 - 1, 2021 - 1900));
moon_age(timelocal(0, 17,  0,  27,  2 - 1, 2021 - 1900));
moon_age(timelocal(0, 48, 11,  28,  3 - 1, 2021 - 1900));
moon_age(timelocal(0, 31, 20,  26,  4 - 1, 2021 - 1900));
moon_age(timelocal(0, 35, 20,  18, 12 - 1, 2021 - 1900));
moon_age(time);