Datatype for account ID

What is the best datatype to store account IDs? Number? String? No! The correct answer is XMLGregorianCalendar .

private XMLGregorianCalendar accountID;


public XMLGregorianCalendar getAccountID()
{
  return accountID;
}


public void setAccountID(XMLGregorianCalendar value)
{
  accountID = value;
}