gistfile1.PL

use strict;
use warnings;
use Test::More;
use DateTime;

my $dt1 = DateTime->now;

{
    t::Util::timeshift(10);
    my $dt2 = DateTime->now;
    cmp_ok $dt2->epoch - $dt1->epoch, '>=', 10;
}

my $dt3 = DateTime->now;
cmp_ok $dt3->epoch - $dt1->epoch, '<=', 10;

done_testing;

package t::Util;
use strict;
use warnings;
use Scope::Upper qw(localize UP);

sub timeshift ($) {
    my $n = shift;
    my $now = DateTime->now;
    localize '&DateTime::now' => sub {
        $now->add(seconds => $n);
    }, UP;
}