Doing the "right thing" over DST shifts

dst_shifts.rb

require 'active_support/version'
require 'active_support/core_ext/time/zones'
require 'active_support/core_ext/time/calculations'
require 'active_support/time_with_zone'

puts "ActiveSupport: #{ActiveSupport::VERSION::STRING}"

Time.zone = 'Eastern Time (US & Canada)'

def show_time user_time
  local = Time.zone.parse user_time
  utc = local.utc
  puts "user: #{user_time} local: #{local}; utc: #{utc}"
end

def show_utc_recording_start_finish(user_start, user_finish)

  local_start = Time.zone.parse(user_start)
  local_finish = Time.zone.parse(user_finish)

  local_delta = local_finish - local_start

  fixed_delta = Time.parse("#{user_finish} UTC") - Time.zone.parse("#{user_start} UTC")

  puts "local delta: #{local_delta}"
  puts "fixed delta: #{fixed_delta}"

  delta = [local_delta, fixed_delta].max

  puts "Delta to use: #{delta}"

  start_utc = local_start.utc
  finish_utc = start_utc + delta
  duration_minutes = ((finish_utc - start_utc) / 60).round
  
  puts "User  #{user_start} - #{user_finish}"
  puts "Local #{local_start} - #{local_start}"
  puts "UTC   #{start_utc} - #{finish_utc} (Duration in minutes: #{duration_minutes})"
end
  
show_time '2013-03-10 01:00:00'
show_time '2013-03-10 02:00:00'
show_time '2013-03-10 03:00:00'

puts

show_time '2013-11-03 01:00:00'
show_time '2013-11-03 02:00:00'
show_time '2013-11-03 03:00:00'

puts

show_utc_recording_start_finish '2013-03-10 01:00:00', '2013-03-10 03:00:00'

puts

show_utc_recording_start_finish '2013-11-03 01:00:00', '2013-11-03 03:00:00'

puts

show_utc_recording_start_finish '2013-12-25 06:00:00', '2013-12-25 07:00:00'