Format date or datetime to string that IB uses.

def formatIBDatetime(dt) -> str:
    """
    Format date or datetime to string that IB uses.
    """
    if not dt:
        s = ''
    elif isinstance(dt, datetime.datetime):
        if dt.tzinfo:
            # convert to local system timezone
            dt = dt.astimezone()
        s = dt.strftime('%Y%m%d %H:%M:%S')
    elif isinstance(dt, datetime.date):
        s = dt.strftime('%Y%m%d 23:59:59')
    else:
        s = dt
    return s