Index: t/42duration_class.t
===================================================================
--- t/42duration_class.t	(revision 0)
+++ t/42duration_class.t	(revision 0)
@@ -0,0 +1,26 @@
+#!/usr/bin/perl -w
+use strict;
+use Test::More tests => 3;
+use DateTime;
+
+do {
+    package DateTime::MySubclass;
+    use base 'DateTime';
+
+    sub duration_class { 'DateTime::Duration::MySubclass' }
+
+    package DateTime::Duration::MySubclass;
+    use base 'DateTime::Duration';
+
+    sub is_my_subclass { 1 }
+};
+
+my $dt = DateTime::MySubclass->now;
+my $delta = $dt - $dt;
+
+isa_ok($delta, 'DateTime::Duration::MySubclass');
+isa_ok($dt + $delta, 'DateTime::MySubclass');
+
+my $delta_days = $dt->delta_days($dt);
+isa_ok($delta_days, 'DateTime::Duration::MySubclass');
+
Index: lib/DateTime.pm
===================================================================
--- lib/DateTime.pm	(revision 4124)
+++ lib/DateTime.pm	(working copy)
@@ -76,6 +76,8 @@
 
 use constant SECONDS_PER_DAY => 86400;
 
+use constant duration_class => 'DateTime::Duration';
+
 my( @MonthLengths, @LeapYearMonthLengths );
 
 BEGIN
@@ -1323,7 +1325,7 @@
     }
 
     return
-        DateTime::Duration->new
+        $dt1->duration_class->new
             ( months      => $months,
 	      days        => $days,
 	      minutes     => $minutes,
@@ -1401,7 +1403,7 @@
     }
 
     return
-        DateTime::Duration->new
+        $self->duration_class->new
             ( seconds     => $seconds,
               nanoseconds => $nanoseconds,
             );
@@ -1431,8 +1433,8 @@
 	      $smaller->_month_length( $smaller->year, $smaller->month ),
             );
 
-    return DateTime::Duration->new( months => $months,
-                                    days   => $days );
+    return $self->duration_class->new( months => $months,
+                                       days   => $days );
 }
 
 sub delta_days
@@ -1442,7 +1444,7 @@
 
     my ( $smaller, $bigger ) = sort( ($self->local_rd_values)[0], ($dt->local_rd_values)[0] );
 
-    DateTime::Duration->new( days => $bigger - $smaller );
+    $self->duration_class->new( days => $bigger - $smaller );
 }
 
 sub delta_ms
@@ -1461,7 +1463,7 @@
     $p{minutes} = $dur->minutes;
     $p{seconds} = $dur->seconds;
 
-    return DateTime::Duration->new(%p);
+    return $self->duration_class->new(%p);
 }
 
 sub _add_overload
@@ -1516,10 +1518,18 @@
     }
 }
 
-sub add { return shift->add_duration( DateTime::Duration->new(@_) ) }
+sub add {
+    my $self = shift;
 
-sub subtract { return shift->subtract_duration( DateTime::Duration->new(@_) ) }
+    return $self->add_duration( $self->duration_class->new(@_) );
+}
 
+sub subtract {
+    my $self = shift;
+
+    return $self->subtract_duration( $self->duration_class->new(@_) );
+}
+
 sub subtract_duration { return $_[0]->add_duration( $_[1]->inverse ) }
 
 {