Round time to next quarter hour with ruby (00,15,30,45)

date_time_helper.rb

module DateTimeHelper
  
  def time_to_next_quarter_hour(time)
    array = time.to_a
    quarter = ((array[1] % 60) / 15.0).ceil
    array[1] = (quarter * 15) % 60
    Time.local(*array) + (quarter == 4 ? 3600 : 0)
  end
  
end

date_time_helper_test.rb

require 'test/unit'

class DateTimeHelperTest < Test::Unit::TestCase

  include DateTimeHelper

  def run_against(hour, minute)
    time = Time.local(2011, 1, 1, hour, minute)
    time_to_next_quarter_hour(time).strftime('%I:%M %p').gsub(/^0/, '')
  end
  
  def test_midnight
    assert_equal '12:00 AM', run_against(00, 00)
  end
  
  def test_on_first_quarter
    assert_equal '12:00 PM', run_against(12, 00)
  end
  
  def test_in_first_quarter
    assert_equal '12:15 PM', run_against(12, 01)
    assert_equal '12:15 PM', run_against(12, 07)
    assert_equal '12:15 PM', run_against(12, 14)
  end
    
  def test_on_second_quarter
    assert_equal '12:15 PM', run_against(12, 15)
  end
  
  def test_in_second_quarter
    assert_equal '12:30 PM', run_against(12, 16)
    assert_equal '12:30 PM', run_against(12, 22)
    assert_equal '12:30 PM', run_against(12, 29)
  end
    
  def test_on_third_quarter
    assert_equal '12:30 PM', run_against(12, 30)
  end
  
  def test_in_third_quarter
    assert_equal '12:45 PM', run_against(12, 31)
    assert_equal '12:45 PM', run_against(12, 37)
    assert_equal '12:45 PM', run_against(12, 44)
  end
  
  def test_on_fourth_quarter
    assert_equal '12:45 PM', run_against(12, 45)
  end
    
  def test_in_fourth_quarter
    assert_equal '1:00 PM', run_against(12, 46)
    assert_equal '1:00 PM', run_against(12, 52)
    assert_equal '1:00 PM', run_against(12, 59)
  end
  
end