This works, too:

use warnings;
use strict;
use feature qw( say );

use DateTime;

my $then = DateTime->new( year => 2013, month => 3, day => 14 );
my $now  = DateTime->now();

say $now->delta_days($then)->delta_days(); # Prints 295
say $then->delta_days($now)->delta_days(); # Also prints 295
[download]

It's actually what the old and probably obsolete FAQ says to do:  use delta_days() twice. See How can I calculate the difference in days between dates? in FAQ: Sample Calculations on the DateTime project wiki.

But much more recently, Dave Rolsky, the author of DateTime, has demonstrated doing it using in_units('days') instead. See Calculating the Difference Between Two Dates in his YAPC::NA 2013 (June 5, 2013) presentation titled A Date with Perl. You can watch Dave's whole presentation at Perl TV and on YouTube

Jim