def _get_local_timezone():
    """ Get local timezone using pytz with environment variable, or dateutil.

    If there is a 'TZ' environment variable, pass it to pandas to use pytz and use it as timezone
    string, otherwise use the special word 'dateutil/:' which means that pandas uses dateutil and
    it reads system configuration to know the system local timezone.

    See also:
    - https://github.com/pandas-dev/pandas/blob/0.19.x/pandas/tslib.pyx#L1753
    - https://github.com/dateutil/dateutil/blob/2.6.1/dateutil/tz/tz.py#L1338
    """
    import os
    return os.environ.get('TZ', 'dateutil/:')
