Problem to update with Timestamp

[O . CHANTEREAU]

Hi everybody

I Have some problems with my update's request.
In fact, I have two transactions

First ----------------------------------------------

        DBUsr old_user = this.findDBUsrByKey(in_num);
             long l = old_user.jdoGetTimeStamp();
             old_user.setUsrNom(in_nom);
             old_user.setUsrPnom(in_pnom);
             old_user.setUsrLogin(in_login);
             old_user.setUsrPwd(in_passwd);
             out_ok = trans.DBUpdate(old_user);

Second ----------------------------------------

public boolean DBUpdate(Object obj) {
    Connexion     c = new Connexion();
    Database      db = c.connect();
    try {
      db.begin();     
      db.update(obj);
      db.commit();
      ok = true;
      db.close();

Message Errors
Erreur Mapping org.exolab.castor.jdo.ObjectModifiedException: Time stamp
mismatched!


Thanks to help me 
Olivier