gistfile1.sql

UPDATE personas 
SET 
  updated_at = (
                  SELECT  personas_23_dic.updated_at 
                  FROM personas_23_dic 
                  WHERE DATE(personas.updated_at) = '2013-12-24' and personas.id = personas_23_dic.id
                ) 
                WHERE EXISTS (
                              SELECT * 
                              FROM personas_23_dic.id = personas.id
                              )