DateTime filter: include nanoseconds, when nonzero

We could show this always, but it would be a change and it seems a
little friendlier to omit zero nanoseconds, since so many duration will
be 0ns, and it won't be interesting.

On the other hand, ignoring some differences isn't a very good way to
present the data!

lib/Data/Printer/Filter/DateTime.pm

@@ -80,8 +80,13 @@ filter 'DateTime', sub {
 filter 'DateTime::Duration', sub {
     my ($obj, $ddp) = @_;
 
-    my @dur    = $obj->in_units(qw(years months days hours minutes seconds));
+    my @dur    = $obj->in_units(qw(years months days hours minutes seconds nanoseconds));
     my $string = "$dur[0]y $dur[1]m $dur[2]d $dur[3]h $dur[4]m $dur[5]s";
+
+    if ($dur[6] > 0) {
+        $string .= " $dur[6]ns";
+    }
+
     return _format( $string, @_ );
 };
 

t/100-filter_datetime.t

@@ -1,6 +1,6 @@
 use strict;
 use warnings;
-use Test::More tests => 21;
+use Test::More tests => 22;
 use Data::Printer::Object;
 
 my $has_timepiece;
@@ -103,14 +103,21 @@ sub test_datetime {
         is( $ddp->parse($d1), '1981-09-29T00:00:00', 'DateTime without TZ data' );
 
         my $diff;
-        skip 'DateTime::Duration not available', 1
+        skip 'DateTime::Duration not available', 2
             unless eval { $diff = $d2 - $d1; $diff && $diff->isa('DateTime::Duration') };
 
         $ddp = Data::Printer::Object->new(
             colored => 0,
             filters => ['DateTime'],
         );
         is( $ddp->parse($diff), '3y 1m 16d 0h 0m 0s', 'DateTime::Duration' );
+
+        my $diff_plus_9ns = $diff->clone->add(nanoseconds => 9);
+        is(
+            $ddp->parse($diff_plus_9ns),
+            '3y 1m 16d 0h 0m 0s 9ns',
+            'DateTime::Duration'
+        );
     };
 }