def _time_to_number(time_string):
    """Change time string to float for easier comparison
    example -> '08:30:00' becomes 8.5
    example -> '08:00:00' becomes 8.0
    example -> '11:30:00' becomes 11.5
    example -> '11:00:00' becomes 11.0
    """

    time_list = time_string.split(':')

    # Check time format
    if len(time_list) != 3:
        raise Exception('Times must be formatted as hh:mm:ss')

    time = float(time_list[0])
    minute = float(time_list[1])

    if minute != 0:
        time = time + 0.5

    return time