diff --git a/addons/mrp/models/mrp_workorder.py b/addons/mrp/models/mrp_workorder.py
index ff9a7979737..f22a39c9af1 100644
--- a/addons/mrp/models/mrp_workorder.py
+++ b/addons/mrp/models/mrp_workorder.py
@@ -454,13 +454,13 @@ class MrpWorkorder(models.Model):
                     raise UserError(_('The planned end date of the work order cannot be prior to the planned start date, please correct this to save the work order.'))
                 if 'duration_expected' not in values and not self.env.context.get('bypass_duration_calculation'):
                     if values.get('date_planned_start') and values.get('date_planned_finished'):
-                        computed_finished_time = self._calculate_date_planned_finished(start_date)
+                        computed_finished_time = workorder._calculate_date_planned_finished(start_date)
                         values['date_planned_finished'] = computed_finished_time
                     elif values.get('date_planned_start'):
-                        computed_duration = self._calculate_duration_expected(date_planned_start=start_date)
+                        computed_duration = workorder._calculate_duration_expected(date_planned_start=start_date)
                         values['duration_expected'] = computed_duration
                     elif values.get('date_planned_finished'):
-                        computed_duration = self._calculate_duration_expected(date_planned_finished=end_date)
+                        computed_duration = workorder._calculate_duration_expected(date_planned_finished=end_date)
                         values['duration_expected'] = computed_duration
                 # Update MO dates if the start date of the first WO or the
                 # finished date of the last WO is update.