Retain UTC offset when using Time.at_with_coercion

The standard Ruby behavior for Time.at is to return the same type of
time when passing an instance of Time as a single argument. Since the
an ActiveSupport::TimeWithZone instance may be a different timezone than
the system timezone and DateTime just understands offsets the best we
can do is to return an instance of Time with the correct offset.

Fixes #11350.

3 files changed, 47 insertions, 3 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 221ceb532a..e29dc636df 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Make `Time.at_with_coercion` retain the second fraction and offset from UTC.
+
+    Fixes #11350
+
+    *Neer Friedman*, *Andrew White*
+
 *   Make `HashWithIndifferentAccess#select` always return the hash, even when
     `Hash#select!` returns `nil`, to allow further chaining.
 
diff --git a/activesupport/lib/active_support/core_ext/time/calculations.rb b/activesupport/lib/active_support/core_ext/time/calculations.rb
index 8270093dc7..20e7e0b303 100644
--- a/activesupport/lib/active_support/core_ext/time/calculations.rb
+++ b/activesupport/lib/active_support/core_ext/time/calculations.rb
@@ -33,10 +33,15 @@ class Time
     # Layers additional behavior on Time.at so that ActiveSupport::TimeWithZone and DateTime
     # instances can be used when called with a single argument
     def at_with_coercion(*args)
-      if args.size == 1 && args.first.acts_like?(:time)
-        at_without_coercion(args.first.to_f)
+      return at_without_coercion(*args) if args.size != 1
+
+      # Time.at can be called with a time or numerical value
+      time_or_number = args.first
+
+      if time_or_number.is_a?(ActiveSupport::TimeWithZone) || time_or_number.is_a?(DateTime)
+        at_without_coercion(time_or_number.to_f).getlocal(time_or_number.utc_offset)
       else
-        at_without_coercion(*args)
+        at_without_coercion(time_or_number)
       end
     end
     alias_method :at_without_coercion, :at
diff --git a/activesupport/test/core_ext/time_ext_test.rb b/activesupport/test/core_ext/time_ext_test.rb
index 1681702bda..63fddd0cfd 100644
--- a/activesupport/test/core_ext/time_ext_test.rb
+++ b/activesupport/test/core_ext/time_ext_test.rb
@@ -700,6 +700,12 @@ class TimeExtCalculationsTest < ActiveSupport::TestCase
     end
   end
 
+  def test_at_with_datetime_maintains_offset
+    with_env_tz 'US/Eastern' do
+      assert_equal 3600, Time.at(DateTime.civil(2000, 1, 1, 0, 0, 0, '+1')).utc_offset
+    end
+  end
+
   def test_at_with_time_with_zone
     assert_equal Time.utc(2000, 1, 1, 0, 0, 0), Time.at(ActiveSupport::TimeWithZone.new(Time.utc(2000, 1, 1, 0, 0, 0), ActiveSupport::TimeZone['UTC']))
 
@@ -711,10 +717,37 @@ class TimeExtCalculationsTest < ActiveSupport::TestCase
     end
   end
 
+  def test_at_with_time_with_zone_maintains_offset
+    with_env_tz 'US/Eastern' do
+      assert_equal 0, Time.at(ActiveSupport::TimeWithZone.new(Time.utc(2000, 1, 1, 0, 0, 0), ActiveSupport::TimeZone['London'])).utc_offset
+      assert_equal 3600, Time.at(ActiveSupport::TimeWithZone.new(Time.utc(2000, 7, 1, 0, 0, 0), ActiveSupport::TimeZone['London'])).utc_offset
+    end
+  end
+
   def test_at_with_time_microsecond_precision
     assert_equal Time.at(Time.utc(2000, 1, 1, 0, 0, 0, 111)).to_f, Time.utc(2000, 1, 1, 0, 0, 0, 111).to_f
   end
 
+  def test_at_with_utc_time
+    with_env_tz 'US/Eastern' do
+      assert_equal Time.utc(2000), Time.at(Time.utc(2000))
+      assert_equal 0, Time.at(Time.utc(2000)).utc_offset
+      assert_equal 'UTC', Time.at(Time.utc(2000)).zone
+    end
+  end
+
+  def test_at_with_local_time
+    with_env_tz 'US/Eastern' do
+      assert_equal Time.local(2000), Time.at(Time.local(2000))
+      assert_equal -18000, Time.at(Time.local(2000)).utc_offset
+      assert_equal 'EST', Time.at(Time.local(2000)).zone
+
+      assert_equal Time.local(2000, 7, 1), Time.at(Time.local(2000, 7, 1))
+      assert_equal -14400, Time.at(Time.local(2000, 7, 1)).utc_offset
+      assert_equal 'EDT', Time.at(Time.local(2000, 7, 1)).zone
+    end
+  end
+
   def test_eql?
     assert_equal true, Time.utc(2000).eql?( ActiveSupport::TimeWithZone.new(Time.utc(2000), ActiveSupport::TimeZone['UTC']) )
     assert_equal true, Time.utc(2000).eql?( ActiveSupport::TimeWithZone.new(Time.utc(2000), ActiveSupport::TimeZone["Hawaii"]) )