:param   date_time: datetime of the date to convert to string
    :param   str_format: str of the format to return the string in
    :return: str of the timestamp

def datetime_to_str(date_time=None, str_format='%Y-%m-%d_%H:%M:%S.fff'):
    """
    :param   date_time: datetime of the date to convert to string
    :param   str_format: str of the format to return the string in
    :return: str of the timestamp
    """
    date_time = date_time or cst_now()
    if DEFAULT_TIMEZONE_AWARE:
        date_time += timedelta(hours=DEFAULT_TIMEZONE)
    ret = date_time.strftime(str_format.split('.f')[0])
    if '.f' in str_format:
        sig_digits = len(str_format.split('.')[-1]) + 1
        ret += str(date_time.microsecond / 1000000.0
                   )[1:].ljust(sig_digits, '0')[:sig_digits]
    return ret