Return correct date in ActiveModel for time to date conversions

time.to_date conversion happens considering leap years
so a conversion of "Day.new({'day(1i)'=>'1', 'day(2i)'=>'1', 'day(3i)'=>'1'})" results in saving the date as Mon, 03 Jan 0001
which might seem weird on the user level, hence falling back to parsing on string level resolves this data mismatch
Fixes #28521

activemodel/CHANGELOG.md

@@ -1,6 +1,21 @@
 ## Rails 5.2.0.beta2 (November 28, 2017) ##
 
-*   No changes.
+*  Return correct date while converting parameters in `value_from_multiparameter_assignment`
+    for `ActiveModel::Type::Date`
+
+    Before:
+
+        Day.new({"day(1i)"=>"1", "day(2i)"=>"1", "day(3i)"=>"1"})
+        => #<Day id: nil, day: "0001-01-03", created_at: nil, updated_at: nil>
+
+    After:
+
+        Day.new({"day(1i)"=>"1", "day(2i)"=>"1", "day(3i)"=>"1"})
+        => #<Day id: nil, day: "0001-01-01", created_at: nil, updated_at: nil>
+
+    Fixes #28521
+
+    *Sayan Chakraborty*
 
 
 ## Rails 5.2.0.beta1 (November 27, 2017) ##

activemodel/lib/active_model/type/date.rb

@@ -17,6 +17,18 @@ module ActiveModel
         value.to_s(:db).inspect
       end
 
+      def is_utc?
+        ::Time.zone_default.nil? || ::Time.zone_default =~ "UTC"
+      end
+
+      def default_timezone
+        if is_utc?
+          :utc
+        else
+          :local
+        end
+      end
+
       private
 
         def cast_value(value)
@@ -49,7 +61,7 @@ module ActiveModel
 
         def value_from_multiparameter_assignment(*)
           time = super
-          time && time.to_date
+          time && new_date(time.year, time.mon, time.mday)
         end
     end
   end

activemodel/test/cases/type/date_test.rb

@@ -15,6 +15,17 @@ module ActiveModel
         date_string = ::Time.now.utc.strftime("%F")
         assert_equal date_string, type.cast(date_string).strftime("%F")
       end
+
+      def test_returns_correct_year
+        type = Type::Date.new
+
+        time = ::Time.utc(1, 1, 1)
+        date = ::Date.new(time.year, time.mon, time.mday)
+
+        values_hash_for_multiparameter_assignment = { 1 => 1, 2 => 1, 3 => 1 }
+
+        assert_equal date, type.cast(values_hash_for_multiparameter_assignment)
+      end
     end
   end
 end