diff --git a/addons/mrp/models/mrp_workorder.py b/addons/mrp/models/mrp_workorder.py
index 1bf27b1a884..56b1c6d9e76 100644
--- a/addons/mrp/models/mrp_workorder.py
+++ b/addons/mrp/models/mrp_workorder.py
@@ -238,30 +238,24 @@ class MrpWorkorder(models.Model):
             workorder.date_finished = workorder.leave_id.date_to
 
     def _set_dates(self):
-        if not self[0].date_start or not self[0].date_finished:
-            if not self.leave_id:
-                return
-            raise UserError(_("It is not possible to unplan one single Work Order. "
-                              "You should unplan the Manufacturing Order instead in order to unplan all the linked operations."))
-        date_from = self[0].date_start
-        date_to = self[0].date_finished
-        to_write = self.env['mrp.workorder']
         for wo in self.sudo():
             if wo.leave_id:
-                to_write |= wo
-            else:
+                if (not wo.date_start or not wo.date_finished):
+                    raise UserError(_("It is not possible to unplan one single Work Order. "
+                              "You should unplan the Manufacturing Order instead in order to unplan all the linked operations."))
+                wo.leave_id.write({
+                    'date_from': wo.date_start,
+                    'date_to': wo.date_finished,
+                })
+            elif wo.date_start and wo.date_finished:
                 wo.leave_id = wo.env['resource.calendar.leaves'].create({
                     'name': wo.display_name,
                     'calendar_id': wo.workcenter_id.resource_calendar_id.id,
-                    'date_from': date_from,
-                    'date_to': date_to,
+                    'date_from': wo.date_start,
+                    'date_to': wo.date_finished,
                     'resource_id': wo.workcenter_id.resource_id.id,
                     'time_type': 'other',
                 })
-        to_write.leave_id.write({
-            'date_from': date_from,
-            'date_to': date_to,
-        })
 
     @api.constrains('blocked_by_workorder_ids')
     def _check_no_cyclic_dependencies(self):