Daylight Saving Time start and end dates in Ruby/Rails

Question

I'm working on a Rails app where I need to find the Daylight Saving Time start and end dates given a specific offset or timezone.

I basically save in my database the timezone offset received from a user's browser( "+3", "-5") and I want to modify it when it changes because of daylight saving time.

I know Time instance variables have the dst? and isdst methods which return true or false if the date stored in them is in the daylight saving time or not.

 > Time.new.isdst
 => true 

But using this to find the Daylight Saving Time beginning and end dates would take too many resources and I also have to do it for each timezone offset I have.

I would like to know a better way of doing this.

Answer 1

Ok, building on what you've said and @dhouty's answer:

You want to be able to feed in an offset and get a set of dates for knowing if there is a DST offset or not. I would recommend ending up with a range made of two DateTime objects, as that is easily used for many purposes in Rails...

require 'tzinfo'

def make_dst_range(offset)

  if dst_end = ActiveSupport::TimeZone[offset].tzinfo.current_period.local_end
     dst_start = ActiveSupport::TimeZone[offset].tzinfo.current_period.local_start
     dst_range = dst_start..dst_end
  else
     dst_range = nil
  end

end

Now you have a method that can do more than just take an offset thanks to the sugar that comes with ActiveSupport. You can do things like:

make_dst_range(-8)
#=> Sun, 08 Mar 2015 03:00:00 +0000..Sun, 01 Nov 2015 02:00:00 +0000

make_dst_range('America/Detroit')
#=> Sun, 08 Mar 2015 03:00:00 +0000..Sun, 01 Nov 2015 02:00:00 +0000

make_dst_range('America/Phoenix')
#=> nil     #returns nil because Phoenix does not observe DST

my_range = make_dst_range(-8)
#=> Sun, 08 Mar 2015 03:00:00 +0000..Sun, 01 Nov 2015 02:00:00 +0000

Today happens to be August 29th so:

my_range.cover?(Date.today)
  #=> true
my_range.cover?(Date.today + 70)
  #=> false
my_range.first
  #=> Sun, 08 Mar 2015 03:00:00 +0000
  #note that this is a DateTime object. If you want to print it use:
my_range.first.to_s
  #=> "2015-03-08T03:00:00+00:00"
my_range.last.to_s
  #=> "2015-11-01T02:00:00+00:00"

ActiveSupport gives you all sorts of goodies for display:

my_range.first.to_formatted_s(:short)
  #=> "08 Mar 03:00"
my_range.first.to_formatted_s(:long)
  #=> "March 08, 2015 03:00"
my_range.first.strftime('%B %d %Y')
   #=> "March 08 2015"

As you can see it's completely doable with just the offset, but as I said, offset doesn't tell you everything, so you might want to grab their actual time zone and store that as a string since the method will happily accept that string and still give you the date range. Even if you are just getting the time offset between your zone and theirs, you can easily figure correct that to the UTC offset:

my_offset = -8
their_offset = -3
utc_offset = my_offset + their_offset

Answer 2

What you are probably looking for is TZInfo::TimezonePeriod. Specifically, the methods local_start/utc_start and local_end/utc_end.

Given a timezone offset, you can get a TZInfo::TimezonePeriod object with

ActiveSupport::TimeZone[-8].tzinfo.current_period

Or if you have a timezone name, you can also get a TZInfo::TimezonePeriod object with

ActiveSupport::TimeZone['America/Los_Angeles'].tzinfo.current_period