Returns the GMT offset for US Eastern Time based on timestamp for the last 30 years

edt_until_1972.rb

module DSTCalculations

  @@dst_years = [
    ["4/3/1966", "10/30/1966"],
    ["4/2/1972", "10/29/1972"],
    ["4/1/1973", "10/28/1973"],
    ["4/7/1974", "10/27/1974"],
    ["4/6/1975", "10/26/1975"],
    ["4/4/1976", "10/31/1976"],
    ["4/3/1977", "10/30/1977"],
    ["4/2/1978", "10/29/1978"],
    ["4/1/1979", "10/28/1979"],
    ["4/6/1980", "10/26/1980"],
    ["4/5/1981", "10/25/1981"],
    ["4/4/1982", "10/31/1982"],
    ["4/3/1983", "10/30/1983"],
    ["4/1/1984", "10/28/1984"],
    ["4/7/1985", "10/27/1985"],
    ["4/6/1986", "10/26/1986"],
    ["4/5/1987", "10/25/1987"],
    ["4/3/1988", "10/30/1988"],
    ["4/2/1989", "10/29/1989"],
    ["4/1/1990", "10/28/1990"],
    ["4/7/1991", "10/27/1991"],
    ["4/5/1992", "10/25/1992"],
    ["4/4/1993", "10/31/1993"],
    ["4/3/1994", "10/30/1994"],
    ["4/2/1995", "10/29/1995"],
    ["4/7/1996", "10/27/1996"],
    ["4/6/1997", "10/26/1997"],
    ["4/5/1998", "10/25/1998"],
    ["4/4/1999", "10/31/1999"],
    ['4/2/2000', '10/29/2000'],
    ['4/1/2001', '10/28/2001'],
    ['4/7/2002', '10/27/2002'],
    ['4/6/2003', '10/26/2003'],
    ['4/4/2004', '10/31/2004'],
    ['4/3/2005', '10/30/2005'],
    ['4/2/2006', '10/29/2006'],
    ['3/11/2007', '11/4/2007'],
    ['3/9/2008', '11/2/2008'],
    ['3/8/2009', '11/1/2009'],
    ['3/14/2010', '11/7/2010'],
    ['3/13/2011', '11/6/2011']
  ]

  def dst_offset(time)
    if ((time.year != 1966 && time.year < 1972) || time.year > 2011)
      puts 'invalid year' and return nil
    end

    idx = (time.year == 1966) ? 0 : (time.year - 1971)
    years =  @@dst_years[idx]

    year_dst_start = DateTime.parse(years.first + " 02:00")
    year_dst_end = DateTime.parse(years.second + " 02:00")

    #puts "time = #{time}, year_dst_start = #{year_dst_start}, year_dst_end = #{year_dst_end}"

    (time >= year_dst_start && time < year_dst_end) ? 4 : 5
  end
end

class DSTCalculationsTest < ActiveSupport::TestCase
  include DSTCalculations

  EDT = 4
  EST = 5

  1972.upto(2011) do |i|
    should "return edt for 6/1/#{i}" do
      assert_equal EDT, dst_offset(DateTime.civil(i, 6, 1, rand(23), rand(59), 0))
    end
  end

  should 'return est for anytime in march pre-2007' do
    2000.upto(2006) do |i|
      assert_equal EST, dst_offset(DateTime.civil(i, 3, rand(15) + 15, rand(23), rand(59), 0))
    end
  end

  # Bush switchover
  should 'return edt for last 3 weeks in march 2007-onwards' do
    2007.upto(2011) do |i|
      assert_equal EDT, dst_offset(DateTime.civil(i, 3, 20, 2, 0, 0))
    end
  end

  # time boundaries
  should 'return est at 1:59 am on the start boundary' do
    assert_equal EST, dst_offset(DateTime.civil(2005, 4, 3, 1, 59, 0))
    assert_equal EST, dst_offset(DateTime.civil(2005, 4, 3, 1, 59, 59))
  end

  should 'return edt at 2:00 am on the start boundary' do
    assert_equal EDT, dst_offset(DateTime.civil(2005, 4, 3, 2, 0, 0))
    assert_equal EDT, dst_offset(DateTime.civil(2005, 4, 3, 3, 0, 0))
  end

  should 'return edt at 1:59 am on the end boundary' do
    assert_equal EDT, dst_offset(DateTime.civil(2009, 11, 1, 1, 59, 0))
    assert_equal EDT, dst_offset(DateTime.civil(2009, 11, 1, 1, 59, 59))
  end

  should 'return est at 2:00 am on the end boundary' do
    assert_equal EST, dst_offset(DateTime.civil(2009, 11, 1, 2, 0, 0))
    assert_equal EST, dst_offset(DateTime.civil(2009, 11, 1, 3, 0, 0))
  end
end