diff --git a/CHANGELOG b/CHANGELOG
index 871362ef6b..ebfe91f2f4 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,5 @@
+* Change days into timedelta overdue
+
 Version 4.0.0 - 2016-05-02
 * Bug fixes (see mercurial logs for details)
 * Add Python3 support
diff --git a/dunning.py b/dunning.py
index b52a2cd968..f4b910751d 100644
--- a/dunning.py
+++ b/dunning.py
@@ -1,12 +1,14 @@
 # This file is part of Tryton.  The COPYRIGHT file at the top level of
 # this repository contains the full copyright notices and license terms.
 from collections import defaultdict
+import datetime
 
 from sql import Null
 from sql.conditionals import Case
 
 from trytond.model import Model, ModelView, ModelSQL, fields, Unique
 from trytond.pyson import If, Eval
+from trytond import backend
 from trytond.transaction import Transaction
 from trytond.wizard import Wizard, StateView, StateAction, StateTransition, \
     Button
@@ -31,7 +33,28 @@ class Level(ModelSQL, ModelView):
     sequence = fields.Integer('Sequence')
     procedure = fields.Many2One('account.dunning.procedure', 'Procedure',
         required=True, select=True)
-    days = fields.Integer('Number of Overdue Days')
+    overdue = fields.TimeDelta('Overdue')
+
+    @classmethod
+    def __register__(cls, module_name):
+        TableHandler = backend.get('TableHandler')
+        cursor = Transaction().connection.cursor()
+        table = TableHandler(cls, module_name)
+        sql_table = cls.__table__()
+
+        super(Level, cls).__register__(module_name)
+
+        # Migration from 4.0: change days into timedelta overdue
+        if table.column_exist('days'):
+            cursor.execute(*sql_table.select(sql_table.id, sql_table.days,
+                    where=sql_table.days != Null))
+            for id_, days in cursor:
+                overdue = datetime.timedelta(days)
+                cursor.execute(*sql_table.update(
+                        [sql_table.days_duration],
+                        [overdue],
+                        where=sql_table.id == id_))
+            table.drop_column('days')
 
     @classmethod
     def __setup__(cls):
@@ -48,8 +71,8 @@ class Level(ModelSQL, ModelView):
             self.procedure.rec_name)
 
     def test(self, line, date):
-        if self.days is not None:
-            return (date - line.maturity_date).days >= self.days
+        if self.overdue is not None:
+            return (date - line.maturity_date) >= self.overdue
 
 _STATES = {
     'readonly': Eval('state') != 'draft',
diff --git a/tests/scenario_account_dunning.rst b/tests/scenario_account_dunning.rst
index 747e0fe23f..f229e8e6f4 100644
--- a/tests/scenario_account_dunning.rst
+++ b/tests/scenario_account_dunning.rst
@@ -78,13 +78,13 @@ Create dunning procedure::
     >>> procedure = Procedure(name='Procedure')
     >>> level = procedure.levels.new()
     >>> level.sequence = 1
-    >>> level.days = 5
+    >>> level.overdue = datetime.timedelta(5)
     >>> level = procedure.levels.new()
     >>> level.sequence = 2
-    >>> level.days = 20
+    >>> level.overdue = datetime.timedelta(20)
     >>> level = procedure.levels.new()
     >>> level.sequence = 3
-    >>> level.days = 40
+    >>> level.overdue = datetime.timedelta(40)
     >>> procedure.save()
 
 Create parties::
diff --git a/view/dunning_level_form.xml b/view/dunning_level_form.xml
index e0b2c0e89d..a50b97ccaf 100644
--- a/view/dunning_level_form.xml
+++ b/view/dunning_level_form.xml
@@ -7,6 +7,6 @@ this repository contains the full copyright notices and license terms. -->
     <label name="sequence"/>
     <field name="sequence"/>
     <newline/>
-    <label name="days"/>
-    <field name="days"/>
+    <label name="overdue"/>
+    <field name="overdue"/>
 </form>
diff --git a/view/dunning_level_list.xml b/view/dunning_level_list.xml
index 4d123e301e..684ecdfdf0 100644
--- a/view/dunning_level_list.xml
+++ b/view/dunning_level_list.xml
@@ -3,5 +3,5 @@
 this repository contains the full copyright notices and license terms. -->
 <tree>
     <field name="procedure" expand="1"/>
-    <field name="days" expand="1"/>
+    <field name="overdue" expand="1"/>
 </tree>
diff --git a/view/dunning_level_list_sequence.xml b/view/dunning_level_list_sequence.xml
index 244f84211a..1297730874 100644
--- a/view/dunning_level_list_sequence.xml
+++ b/view/dunning_level_list_sequence.xml
@@ -3,5 +3,5 @@
 this repository contains the full copyright notices and license terms. -->
 <tree sequence="sequence">
     <field name="procedure" expand="1"/>
-    <field name="days" expand="1"/>
+    <field name="overdue" expand="1"/>
 </tree>