clock.rb

def speak_hour(hour)
  hour = 0 if hour == 24

  {
    0 => "midnight",
    1 => "one",
    2 => "two",
    3 => "three",
    4 => "four",
    6 => "six",
    7 => "seven",
    8 => "eight",
    10 => "ten",
    11 => "eleven",
    12 => "twelve"
    #...
  }.fetch(hour)
end

def speak_minutes(minutes)
  {
    5 => "five",
    10 => "ten",
    15 => "quarter",
    20 => "twenty",
    25 => "twenty five",
    32 => "thirty two"
  }.fetch(minutes)
end

def speak(time)
  hour, minutes = time.split(":").map(&:to_i)

  if hour.zero? and minutes.zero?
    speak_hour(hour)
  elsif hour == 12 and minutes.zero?
    "noon"
  elsif minutes.zero?
    "#{speak_hour(hour)} o'clock"
  elsif minutes < 30
    "#{speak_minutes(minutes)} past #{speak_hour(hour)}"
  elsif minutes == 30
    "half past #{speak_hour(hour)}"
  elsif minutes > 30 and !minutes.modulo(5).zero?
    "#{speak_hour(hour)} #{speak_minutes(minutes)}"
  else
    "#{speak_minutes(60 - minutes)} to #{speak_hour(hour + 1)}"
  end
end

clock_test.rb

require 'minitest/autorun'
require_relative 'clock'

class ClockTest < Minitest::Test
  def test_works_with_1_00
    assert_equal "one o'clock", speak('1:00')
  end

  def test_works_with_2_05
    assert_equal "five past two", speak('2:05')
  end

  def test_works_with_3_10
    assert_equal "ten past three", speak('3:10')
  end

  def test_works_with_4_15
    assert_equal "quarter past four", speak('4:15')
  end

  def test_works_with_6_25
    assert_equal "twenty five past six", speak('6:25')
  end

  def test_works_with_6_32
    assert_equal "six thirty two", speak('6:32')
  end

  def test_works_with_7_30
    assert_equal "half past seven", speak('7:30')
  end

  def test_works_with_7_35
    assert_equal "twenty five to eight", speak('7:35')
  end

  def test_works_with_7_40
    assert_equal "twenty to eight", speak('7:40')
  end

  def test_works_with_9_45
    assert_equal "quarter to ten", speak('9:45')
  end

  def test_works_with_10_50
    assert_equal "ten to eleven", speak('10:50')
  end

  def test_works_with_11_55
    assert_equal "five to twelve", speak('11:55')
  end

  def test_works_with_00_00
    assert_equal "midnight", speak('00:00')
  end

  def test_works_with_12_00
    assert_equal "noon", speak('12:00')
  end

  def test_works_with_23_55
    assert_equal "noon", speak('23:55')
  end
end