TimeWithZone#- gives correct result with wrapped DateTime, and with DateTime argument

activesupport/CHANGELOG

 *2.3.0 [Edge]*
 
+* TimeWithZone#- gives correct result with wrapped DateTime, and with DateTime argument [Geoff Buesing]
+
 * Updated i18n gem to version 0.1.1 #1635 [Yaroslav Markin]
 
 * Add :allow_nil option to delegate. #1127 [Sergio Gil]

activesupport/lib/active_support/time_with_zone.rb

@@ -199,7 +199,7 @@ def -(other)
       # If we're subtracting a Duration of variable length (i.e., years, months, days), move backwards from #time,
       # otherwise move backwards #utc, for accuracy when moving across DST boundaries
       if other.acts_like?(:time)
-        utc - other
+        utc.to_f - other.to_f
       elsif duration_of_variable_length?(other)
         method_missing(:-, other)
       else

activesupport/test/core_ext/time_with_zone_test.rb

@@ -256,6 +256,15 @@ def test_minus_with_time_with_zone
     twz2 = ActiveSupport::TimeWithZone.new( Time.utc(2000, 1, 2), ActiveSupport::TimeZone['UTC'] )
     assert_equal  86_400.0,  twz2 - twz1
   end
+  
+  def test_minus_with_datetime
+    assert_equal  86_400.0,  ActiveSupport::TimeWithZone.new( Time.utc(2000, 1, 2), ActiveSupport::TimeZone['UTC'] ) - DateTime.civil(2000, 1, 1)
+  end
+  
+  def test_minus_with_wrapped_datetime
+    assert_equal  86_400.0,  ActiveSupport::TimeWithZone.new( DateTime.civil(2000, 1, 2), ActiveSupport::TimeZone['UTC'] ) - Time.utc(2000, 1, 1)
+    assert_equal  86_400.0,  ActiveSupport::TimeWithZone.new( DateTime.civil(2000, 1, 2), ActiveSupport::TimeZone['UTC'] ) - DateTime.civil(2000, 1, 1)
+  end
 
   def test_plus_and_minus_enforce_spring_dst_rules
     silence_warnings do # silence warnings raised by tzinfo gem