def check_tz(time_index):
    """
    Check datetime index for timezone, if None set to UTC

    Parameters
    ----------
    time_index : pandas.DatatimeIndex
        DatetimeIndex to check timezone for

    Returns
    -------
    time_index : pandas.DatatimeIndex
        Updated DatetimeIndex with timezone set
    """
    if not time_index.tz:
        time_index = time_index.tz_localize('utc')

    return time_index