diff --git a/addons/project/models/project.py b/addons/project/models/project.py
index 891e1ab8fa2..073b0781707 100644
--- a/addons/project/models/project.py
+++ b/addons/project/models/project.py
@@ -292,6 +292,14 @@ class Task(models.Model):
     _mail_post_access = 'read'
     _order = "priority desc, sequence, date_start, name, id"
 
+    @api.model
+    def default_get(self, field_list):
+        """ Set 'date_assign' if user_id is set. """
+        result = super(Task, self).default_get(field_list)
+        if 'user_id' in result:
+            result['date_assign'] = fields.Datetime.now()
+        return result
+
     def _get_default_partner(self):
         if 'default_project_id' in self.env.context:
             default_project_id = self.env['project.project'].browse(self.env.context['default_project_id'])