def _get_log_time_scale(units):
    """Retrieves the ``log10()`` of the scale factor for a given time unit.

    Args:
        units (str): String specifying the units
            (one of ``'fs'``, ``'ps'``, ``'ns'``, ``'us'``, ``'ms'``, ``'sec'``).

    Returns:
        The the ``log10()`` of the scale factor for the time unit.
    """
    scale = {'fs': -15, 'ps': -12, 'ns': -9, 'us': -6, 'ms': -3, 'sec': 0}
    units_lwr = units.lower()
    if units_lwr not in scale:
        raise ValueError('Invalid unit ({0}) provided'.format(units))
    else:
        return scale[units_lwr]
