#!/usr/bin/perl

use utf8; use 5.022; use strict; use List::Util qw(max); use Date::Calc qw(Delta_Days);

say "FINISHED";
my $pause_here = <STDIN>;

##</code><code>##

#!/usr/bin/perl

use utf8; use 5.022;
use strict; use List::Util qw(max); use Date::Calc qw(Delta_Days);

say "FINISHED";
my $pause_here = <STDIN>;

##</code><code>##

#!/usr/bin/perl

use utf8; use 5.022; use strict; use List::Util qw(max);

say "FINISHED";
my $pause_here = <STDIN>;

##</code><code>##

use utf8;
use 5.022;
use strict;
use warnings;

for (my $year=1970; $year<=2100; $year++)
{
    for (my $month=1; $month<=12; $month++)
    {
        for (my $day=1; $day<=31; $day++)
        {
            next if 
            (
                 ($month == 2 && ($day == 30 || $day == 31))
              || ($month == 2 && $day == 29 && ($year%4 != 0 || ($year%100 == 0 && $year%400 != 0)))
              || ($month == 4 && $day == 31)
              || ($month == 6 && $day == 31)
              || ($month == 9 && $day == 31)
              || ($month == 11 && $day == 31)
            );
            
            my $date1 = $month. "/" . $day . "/" . $year;
            
            my $date2 = $month . "/" . ($day+1) . "/" . $year;
            $date2 = ($month+1) . "/1/" . $year if
            (
                 ($day+1 == 32)
              || ($month == 2 && ($day+1 == 30 || $day+1 == 31))
              || ($month == 2 && $day+1 == 29 && ($year%4 != 0 || ($year%100 == 0 && $year%400 != 0)))
              || ($month == 4 && $day+1 == 31)
              || ($month == 6 && $day+1 == 31)
              || ($month == 9 && $day+1 == 31)
              || ($month == 11 && $day+1 == 31)
            );
            $date2 = "1/1/" . ($year+1) if ($month+1 == 13  && $day+1 == 32);
            
            my $difference = days_diff($date2, $date1);
            
            say "$date1\t$date2\t$difference" if ($difference != 1);
        }
    }
}

say "Hit Enter to quit.";
my $pause = <STDIN>;

sub days_diff
{
    my ($month1, $day1, $year1) = split /\//, $_[0];
    my ($month2, $day2, $year2) = split /\//, $_[1];
    
    # The Date::Calc module function "Delta_Days" was used for this and worked great, but it would not compile in to an .exe using pp.
    # use Date::Calc qw(Delta_Days); # use is here for convenient copy/paste
    # return Delta_Days($year2, $month2, $day2, $year1, $month1, $day1);
    
    # This crude (not precise over long timeframes or short ones spanning the end of February and March, but this was fine for the application) method was used for a long time, 
    # but it fails (in an unacceptable way) when subtracting a day that falls on the 1st of the next month from a day that falls on the 31st the prior month (gives -0, not -1),
    # return sprintf("%.0f", ($year1-$year2)*365.25+($month1-$month2)*365.25/12+($day1-$day2));
    
    # Here a Gergorian calendar date to Julian day number conversion is used on both dates and then they are subtracted.
    # https://en.wikipedia.org/wiki/Julian_day#Converting_Julian_or_Gregorian_calendar_date_to_Julian_day_number
    use POSIX qw(floor); # use is here for convenient copy/paste
    my $a1 = floor((14-$month1)/12);
    my $a2 = floor((14-$month2)/12);
    my $y1 = $year1 + 4800 - $a1;
    my $y2 = $year2 + 4800 - $a2;
    my $m1 = $month1 + 12*$a1 - 3;
    my $m2 = $month2 + 12*$a2 - 3;
    my $JDN1 = $day1 + floor((153*$m1+2)/5) + 365*$y1 + floor($y1/4) - floor($y1/100) + floor($y1/400) - 32045;
    my $JDN2 = $day2 + floor((153*$m2+2)/5) + 365*$y2 + floor($y2/4) - floor($y2/100) + floor($y2/400) - 32045;
    return $JDN1-$JDN2;
}