#!/usr/bin/env perl
use warnings;
use strict;
use DateTime;

my $dt_customepoch = DateTime->new(
	year=>2000, month=>1, day=>1,
	hour=>12, minute=>0, second=>0,
	time_zone=>'UTC' );
print "dt_customepoch: ",
	$dt_customepoch->strftime('%b %e %Y, %T %Z'), "\n";
my $s_customepoch = $dt_customepoch->epoch;
print " s_customepoch: $s_customepoch\n";

### Obtaining DateTime object from a value expressed in
# seconds since the custom epoch (let's call it "sce")

my $sce_one = 128072700; # Thu Jan 22 19:45:00 2004
print "   sce_one: $sce_one\n";
my $dt_one = DateTime->from_epoch(
	# Note the addition here
	epoch => $s_customepoch + $sce_one,
	# from_epoch defaults to UTC, but we'll be explicit
	time_zone=>'UTC' );
print "    dt_one: ", $dt_one->strftime('%b %e %Y, %T %Z'),
	" (epoch=", $dt_one->epoch, ")\n";

#### Doing math in the "seconds since custom epoch" domain

# Add 3 years, 21 days*, -4 hours, 11 minutes
# * adjusted +1 for 2004 being leap year
my $s_offset = 60*60*24*365*3 + 60*60*24*(21+1)
	- 60*60*4 + 60*11;
print "  s_offset: $s_offset\n";
my $sce_two = $sce_one + $s_offset;
print "   sce_two: $sce_two\n";

my $dt_two = DateTime->from_epoch( time_zone=>'UTC',
	epoch => $s_customepoch + $sce_two );
print "    dt_two: ", $dt_two->strftime('%b %e %Y, %T %Z'),
	" (epoch=", $dt_two->epoch, ")\n";

### Doing the same math in the DateTime domain

my $dt_three = $dt_one->clone->add(
	# Note how no adjustment for the leap year is needed
	years=>3, days=>21, hours=>-4, minutes=>11);
print "  dt_three: ", $dt_three->strftime('%b %e %Y, %T %Z'),
	" (epoch=", $dt_three->epoch, ")\n";

#### Obtaining "seconds from custom epoch"
# using regular subtraction vs. DateTime math

my $sce_threea = $dt_three->epoch - $s_customepoch;
print "sce_threea: $sce_threea\n";

my $sce_threeb = $dt_three
	->subtract_datetime_absolute($dt_customepoch)
	->in_units('seconds');
# Leap second (Dec 31, 2005 23:59:60 UTC) is considered! 
print "sce_threeb: $sce_threeb\n";

__END__

dt_customepoch: Jan  1 2000, 12:00:00 UTC
 s_customepoch: 946728000
   sce_one: 128072700
    dt_one: Jan 22 2004, 19:45:00 UTC (epoch=1074800700)
  s_offset: 96495060
   sce_two: 224567760
    dt_two: Feb 12 2007, 15:56:00 UTC (epoch=1171295760)
  dt_three: Feb 12 2007, 15:56:00 UTC (epoch=1171295760)
sce_threea: 224567760
sce_threeb: 224567761