adding days to date

Hi,

I came across some unexpected behaviour in datetime. In the following
script, I first define $date1. Then I set $day1 = $date1. Then I add 2 days
to $day1. Why does $date1 also get incremented?

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

use DateTime;

my $date1 = DateTime->new(year => 2007,
             month => 12,
             day => 23);

my $day1 = $date1;

$day1->add(days => 2);

print "day1 = ",$day1,"\n";   # gives me 2007-12-25 correctly.
print "date1 = ",$date1,"\n";  # why does $date1 also change to 2007-12-25?


Jagdish Eashwar