def _stringify_time_unit(unit, value) -> str:
    """
    Returns a string representing a certain unit of time.
    For example:
    >>> _stringify_time_unit('hours', 9)
    '9 hours'
    >>> _stringify_time_unit('minutes', 0)
    'less than a minute'
    """
    if unit == 'seconds' and value == 0:
        return '0 seconds'
    elif value == 1:
        # Use singular noun if there is only 1 unit
        return f'{value} {unit[:-1]}'
    elif value == 0:
        # Use 'less than a' if there are 0 units
        return f'less than a {unit[:-1]}'
    else:
        return f'{value} {unit}'