Daylight Savings Time Finder

daylight_savings_time_finder.ruby

def daylight_savings_time_starts(year)
  date = Date.new(year, 3, 1)
  return Date.new(year, 3, 8) if date.wday == 0
  return Date.new(year, 3, (7 - date.wday + 7 + 1))  
end

def daylight_savings_time_stops(year)
  date = Date.new(year, 11, 1)
  return date if date.wday == 0
  return Date.new(year, 11, (7 - date.wday + 1))  
end

# Daylight Saving Time (United States) 2013 began at 2:00 AM on
# Sunday, March 10
# and ended at 2:00 AM on
# Sunday, November 3
daylight_savings_time_starts(2013) # => #<Date: 2013-03-10 ((2456362j,0s,0n),+0s,2299161j)>
daylight_savings_time_stops(2013) # => #<Date: 2013-11-03 ((2456600j,0s,0n),+0s,2299161j)>

#Daylight Saving Time (United States) 2014 began at 2:00 AM on
#Sunday, March 9
#and ended at 2:00 AM on
#Sunday, November 2
daylight_savings_time_starts(2014) # => #<Date: 2014-03-09 ((2456726j,0s,0n),+0s,2299161j)>
daylight_savings_time_stops(2014) # => #<Date: 2014-11-02 ((2456964j,0s,0n),+0s,2299161j)>


# Daylight Saving Time (United States) 2015 began at 2:00 AM on
# Sunday, March 8
# and ends at 2:00 AM on
# Sunday, November 1
daylight_savings_time_starts(2015) # => #<Date: 2015-03-08 ((2457090j,0s,0n),+0s,2299161j)>
daylight_savings_time_stops(2015) # => #<Date: 2015-11-01 ((2457328j,0s,0n),+0s,2299161j)>

def calculate_offset_for_dst(epoch_time)
  date = Time.at(epoch_time).to_date
  if daylight_savings_time_starts(date.year) <= date && date < daylight_savings_time_stops(date.year)
    "-06:00"
  else
    "-05:00"
  end
end
epoch = 1425811100
Time.at(epoch).to_date # => #<Date: 2015-03-08 ((2457090j,0s,0n),+0s,2299161j)>
calculate_offset_for_dst(epoch) # => "-06:00"

epoch = 1446411111
Time.at(epoch).to_date # => #<Date: 2015-11-01 ((2457328j,0s,0n),+0s,2299161j)>
calculate_offset_for_dst(epoch) # => "-05:00"