diff --git a/addons/mrp/models/mrp_workorder.py b/addons/mrp/models/mrp_workorder.py
index f22a39c9af1..49907a0bb57 100644
--- a/addons/mrp/models/mrp_workorder.py
+++ b/addons/mrp/models/mrp_workorder.py
@@ -448,19 +448,16 @@ class MrpWorkorder(models.Model):
                     workorder.leave_id.resource_id = self.env['mrp.workcenter'].browse(values['workcenter_id']).resource_id
         if 'date_planned_start' in values or 'date_planned_finished' in values:
             for workorder in self:
-                start_date = fields.Datetime.to_datetime(values.get('date_planned_start')) or workorder.date_planned_start
-                end_date = fields.Datetime.to_datetime(values.get('date_planned_finished')) or workorder.date_planned_finished
+                start_date = fields.Datetime.to_datetime(values.get('date_planned_start', workorder.date_planned_start))
+                end_date = fields.Datetime.to_datetime(values.get('date_planned_finished', workorder.date_planned_finished))
                 if start_date and end_date and start_date > end_date:
                     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 = workorder._calculate_date_planned_finished(start_date)
                         values['date_planned_finished'] = computed_finished_time
-                    elif values.get('date_planned_start'):
-                        computed_duration = workorder._calculate_duration_expected(date_planned_start=start_date)
-                        values['duration_expected'] = computed_duration
-                    elif values.get('date_planned_finished'):
-                        computed_duration = workorder._calculate_duration_expected(date_planned_finished=end_date)
+                    elif start_date and end_date:
+                        computed_duration = workorder._calculate_duration_expected(date_planned_start=start_date, 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.