4 files changed, 30 insertions, 3 deletions
diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index adaeb99c87..ba450b85c4 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,15 @@
+*   Maintain the current timezone when calling `change` during DST overlap
+
+    Currently if a time is changed during DST overlap in the autumn then the method
+    `period_for_local` will return the DST period. However if the original time is
+    not DST then this can be surprising and is not what is generally wanted. This
+    commit changes that behavior to maintain the current period if it's in the list
+    of periods returned by `periods_for_local`.
+
+    Fixes #12163.
+
+    *Andrew White*
+
 *   Added `Hash#compact` and `Hash#compact!` for removing items with nil value from hash.
 
     *Celestino Gomes*
diff --git a/activesupport/lib/active_support/time_with_zone.rb b/activesupport/lib/active_support/time_with_zone.rb
index 50db7da9d9..d47aee00f4 100644
--- a/activesupport/lib/active_support/time_with_zone.rb
+++ b/activesupport/lib/active_support/time_with_zone.rb
@@ -45,7 +45,7 @@ module ActiveSupport
 
     def initialize(utc_time, time_zone, local_time = nil, period = nil)
       @utc, @time_zone, @time = utc_time, time_zone, local_time
-      @period = @utc ? period : get_period_and_ensure_valid_local_time
+      @period = @utc ? period : get_period_and_ensure_valid_local_time(period)
     end
 
     # Returns a Time or DateTime instance that represents the time in +time_zone+.
@@ -292,6 +292,12 @@ module ActiveSupport
       end
     end
 
+    def change(options)
+      new_time = time.change(options)
+      periods = time_zone.periods_for_local(new_time)
+      self.class.new(nil, time_zone, new_time, periods.include?(period) ? period : nil)
+    end
+
     %w(year mon month day mday wday yday hour min sec usec nsec to_date).each do |method_name|
       class_eval <<-EOV, __FILE__, __LINE__ + 1
         def #{method_name}    # def month
@@ -367,12 +373,12 @@ module ActiveSupport
     end
 
     private
-      def get_period_and_ensure_valid_local_time
+      def get_period_and_ensure_valid_local_time(period)
         # we don't want a Time.local instance enforcing its own DST rules as well,
         # so transfer time values to a utc constructor if necessary
         @time = transfer_time_values_to_utc_constructor(@time) unless @time.utc?
         begin
-          @time_zone.period_for_local(@time)
+          period || @time_zone.period_for_local(@time)
         rescue ::TZInfo::PeriodNotFound
           # time is in the "spring forward" hour gap, so we're moving the time forward one hour and trying again
           @time += 1.hour
diff --git a/activesupport/lib/active_support/values/time_zone.rb b/activesupport/lib/active_support/values/time_zone.rb
index beaac42fa1..eb785d46ce 100644
--- a/activesupport/lib/active_support/values/time_zone.rb
+++ b/activesupport/lib/active_support/values/time_zone.rb
@@ -352,6 +352,10 @@ module ActiveSupport
       tzinfo.period_for_local(time, dst)
     end
 
+    def periods_for_local(time) #:nodoc:
+      tzinfo.periods_for_local(time)
+    end
+
     def self.find_tzinfo(name)
       TZInfo::TimezoneProxy.new(MAPPING[name] || name)
     end
diff --git a/activesupport/test/core_ext/time_with_zone_test.rb b/activesupport/test/core_ext/time_with_zone_test.rb
index 5494824a40..43fe572bfc 100644
--- a/activesupport/test/core_ext/time_with_zone_test.rb
+++ b/activesupport/test/core_ext/time_with_zone_test.rb
@@ -511,6 +511,11 @@ class TimeWithZoneTest < ActiveSupport::TestCase
     assert_equal "Fri, 31 Dec 1999 19:00:30 EST -05:00", @twz.change(:sec => 30).inspect
   end
 
+  def test_change_at_dst_boundary
+    twz = ActiveSupport::TimeWithZone.new(Time.at(1319936400).getutc, ActiveSupport::TimeZone['Madrid'])
+    assert_equal twz, twz.change(:min => 0)
+  end
+
   def test_advance
     assert_equal "Fri, 31 Dec 1999 19:00:00 EST -05:00", @twz.inspect
     assert_equal "Mon, 31 Dec 2001 19:00:00 EST -05:00", @twz.advance(:years => 2).inspect