Standardise the return value of `to_time`

This commit standardises the return value of `to_time` to an instance
of `Time` in the local system timezone, matching the Ruby core and
standard library behavior.

The default form for `String#to_time` has been changed from :utc to
:local but research seems to suggest the latter is the more common form.

Also fix an edge condition with `String#to_time` where the string has
a timezone offset in it and the mode is :local. e.g:

  # Before:
  >> "2000-01-01 00:00:00 -0500".to_time(:local)
  => 2000-01-01 05:00:00 -0500

  # After:
  >> "2000-01-01 00:00:00 -0500".to_time(:local)
  => 2000-01-01 00:00:00 -0500

Closes #2453

1 files changed, 6 insertions, 2 deletions

diff --git a/activesupport/test/core_ext/date_ext_test.rb b/activesupport/test/core_ext/date_ext_test.rb
index ec47d0632c..f3fa96ec6f 100644
--- a/activesupport/test/core_ext/date_ext_test.rb
+++ b/activesupport/test/core_ext/date_ext_test.rb
@@ -33,8 +33,12 @@ class DateExtCalculationsTest < ActiveSupport::TestCase
   end
 
   def test_to_time
-    assert_equal Time.local(2005, 2, 21), Date.new(2005, 2, 21).to_time
-    assert_equal Time.local(2039, 2, 21), Date.new(2039, 2, 21).to_time
+    with_env_tz 'US/Eastern' do
+      assert_equal Time, Date.new(2005, 2, 21).to_time.class
+      assert_equal Time.local(2005, 2, 21), Date.new(2005, 2, 21).to_time
+      assert_equal Time.local(2005, 2, 21).utc_offset, Date.new(2005, 2, 21).to_time.utc_offset
+    end
+
     silence_warnings do
       0.upto(138) do |year|
         [:utc, :local].each do |format|