cannot get message's date

[daijun]

I'm trying to get a message's date. 
 But both getSenderDate() and getReceivedDate() are null.
 My code is as below:
       Store store = session.getStore();
      store.connect();
      Folder root = store.getDefaultFolder();
      Folder inbox = root.getFolder("inbox");
      inbox.open(Folder.READ_WRITE);
      Message[] msgs = inbox.getMessages();
      I can getFrom and getContent... but when I was trying to get date by:
       msgs[i].getSentDate()
      msg[i].getReceivedDate()
       I got null. 
       How can I get the message's date?
  
 Thank you in advance.
 Best Regards
  
 D. J.