Ensure duration parsing is consistent across DST changes

Previously `ActiveSupport::Duration.parse` used `Time.current` and
`Time#advance` to calculate the number of seconds in the duration
from an arbitrary collection of parts. However as `advance` tries to
be consistent across DST boundaries this meant that either the
duration was shorter or longer depending on the time of year.

This was fixed by using an absolute reference point in UTC which
isn't subject to DST transitions. An arbitrary date of Jan 1st, 2000
was chosen for no other reason that it seemed appropriate.

Additionally, duration parsing should now be marginally faster as we
are no longer creating instances of `ActiveSupport::TimeWithZone`
every time we parse a duration string.

Fixes #26941.

3 files changed, 54 insertions, 2 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 83ff80e31a..fac91c31da 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,23 @@
+*   Ensure duration parsing is consistent across DST changes
+
+    Previously `ActiveSupport::Duration.parse` used `Time.current` and
+    `Time#advance` to calculate the number of seconds in the duration
+    from an arbitrary collection of parts. However as `advance` tries to
+    be consistent across DST boundaries this meant that either the
+    duration was shorter or longer depending on the time of year.
+
+    This was fixed by using an absolute reference point in UTC which
+    isn't subject to DST transitions. An arbitrary date of Jan 1st, 2000
+    was chosen for no other reason that it seemed appropriate.
+
+    Additionally, duration parsing should now be marginally faster as we
+    are no longer creating instances of `ActiveSupport::TimeWithZone`
+    every time we parse a duration string.
+
+    Fixes #26941.
+
+    *Andrew White*
+
 *   Use `Hash#compact` and `Hash#compact!` from Ruby 2.4. Old Ruby versions
     will continue to get these methods from Active Support as before.
 
diff --git a/activesupport/lib/active_support/duration.rb b/activesupport/lib/active_support/duration.rb
index 862dabb1e8..82322291d0 100644
--- a/activesupport/lib/active_support/duration.rb
+++ b/activesupport/lib/active_support/duration.rb
@@ -7,6 +7,8 @@ module ActiveSupport
   #
   #   1.month.ago       # equivalent to Time.now.advance(months: -1)
   class Duration
+    EPOCH = ::Time.utc(2000)
+
     attr_accessor :value, :parts
 
     autoload :ISO8601Parser,     "active_support/duration/iso8601_parser"
@@ -140,8 +142,7 @@ module ActiveSupport
     # If invalid string is provided, it will raise +ActiveSupport::Duration::ISO8601Parser::ParsingError+.
     def self.parse(iso8601duration)
       parts = ISO8601Parser.new(iso8601duration).parse!
-      time  = ::Time.current
-      new(time.advance(parts) - time, parts)
+      new(EPOCH.advance(parts) - EPOCH, parts)
     end
 
     # Build ISO 8601 Duration string for this duration.
diff --git a/activesupport/test/core_ext/duration_test.rb b/activesupport/test/core_ext/duration_test.rb
index 53575272e3..c8e8b8a350 100644
--- a/activesupport/test/core_ext/duration_test.rb
+++ b/activesupport/test/core_ext/duration_test.rb
@@ -322,4 +322,35 @@ class DurationTest < ActiveSupport::TestCase
       assert_equal time + duration, time + ActiveSupport::Duration.parse(duration.iso8601), pattern.inspect
     end
   end
+
+  def test_iso8601_parsing_across_spring_dst_boundary
+    with_env_tz eastern_time_zone do
+      with_tz_default "Eastern Time (US & Canada)" do
+        travel_to Time.utc(2016, 3, 11) do
+          assert_equal 604800, ActiveSupport::Duration.parse("P7D").to_i
+          assert_equal 604800, ActiveSupport::Duration.parse("P1W").to_i
+        end
+      end
+    end
+  end
+
+  def test_iso8601_parsing_across_autumn_dst_boundary
+    with_env_tz eastern_time_zone do
+      with_tz_default "Eastern Time (US & Canada)" do
+        travel_to Time.utc(2016, 11, 4) do
+          assert_equal 604800, ActiveSupport::Duration.parse("P7D").to_i
+          assert_equal 604800, ActiveSupport::Duration.parse("P1W").to_i
+        end
+      end
+    end
+  end
+
+  private
+    def eastern_time_zone
+      if Gem.win_platform?
+        "EST5EDT"
+      else
+        "America/New_York"
+      end
+    end
 end