def _convert_month_to_date_str(month_str):
    """
    Convert string of month format to date format. If it is not a month
    format, return the value without converting.

    Parameters
    ----------
    month_str : str
        String to be converted. e.g., 2019-01

    Returns
    -------
    date_str : str
        String converted to date format. e.g., 2019-01-01
    """
    if len(month_str) != 7:
        return month_str
    date_str = month_str + '-01'
    return date_str