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;
}