subtract_datetime() calculates the difference in days & months but you choose only the day difference and ignore the month difference. And, delta_days(), per pod of installed version here, takes a DateTime object unlike your usage. in_units() seems screwy until one gets the doc linked there.

use strict; use warnings;

use Data::Dumper;
local $Data::Dumper::Indent   = 2 ;
local $Data::Dumper::Pad      = ' ' ;
local $Data::Dumper::Deepcopy = 1 ;

use DateTime;

my %base =
  ( 'hour' => 0 , 'minute'=> 0 , 'second' => 0
  , 'time_zone' => +0000
  );

my $ref_date =
  DateTime->new( %base , 'year'  => 2013 , 'month' => 1 , 'day'  => 1 
+);

my @date =
  ( DateTime->new( %base , 'year' => 2013 , 'month' => 1 , 'day' => 31
+ )
  , DateTime->new( %base , 'year' => 2013 , 'month' => 2 , 'day' =>  3
+ )
  , DateTime->new( %base , 'year' => 2014 , 'month' => 1 , 'day' =>  2
+ )
  );

for my $date ( @date )
{
  for my $method ( \&via_pcouderc , \&via_choroba )
  {
    printf "%s :: %s ...\n" , map $_->iso8601(), $ref_date , $date;
    $method->( $ref_date , $date );
    print "\n";
  }
  print "\n";
}

exit;


sub via_pcouderc
{
  my ( $one , $two ) = @_;

  my $obj = $two->subtract_datetime( $one );
  return show( $obj , $obj->delta_days() , ( caller(0) )[3] );
}

sub via_choroba
{
  my ( $one , $two ) = @_;

  my $obj = $two->delta_days( $one );
  return show( $obj , $obj->in_units('days') , ( caller(0) )[3] );
}

sub show
{
  my ( $interval , $result , $sub ) = @_;

  printf "%s => %s\n => %s\n" , $sub , $result , Dumper( $interval );
  return;
}

__END__
[download]

Could somebody please either hide the Perl code in "readmore" tags and remove "readmore" from the output; or hide both parts? Rational is that output is more important, for the OP simply needs to spend quality time with DateTime to understand it.