Merge pull request #15421 from gchan/time_with_zone_precision

Fixed `ActiveSupport::TimeWithZone#-` so precision is not unnecessarily lost

3 files changed, 36 insertions, 1 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 8a9d31a7bb..871e1705f7 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,23 @@
+*   Fixed `ActiveSupport::TimeWithZone#-` so precision is not unnecessarily lost
+    when working with objects with a nanosecond component.
+
+    `ActiveSupport::TimeWithZone#-` should return the same result as if we were
+    using `Time#-`:
+
+        Time.now.end_of_day - Time.now.beginning_of_day #=> 86399.999999999
+
+    Before:
+
+        Time.zone.now.end_of_day.nsec #=> 999999999
+        Time.zone.now.end_of_day - Time.zone.now.beginning_of_day #=> 86400.0
+
+    After:
+
+        Time.zone.now.end_of_day - Time.zone.now.beginning_of_day
+        #=> 86399.999999999
+
+    *Gordon Chan*
+
 *   Fixed precision error in NumberHelper when using Rationals.
 
     before:
diff --git a/activesupport/lib/active_support/time_with_zone.rb b/activesupport/lib/active_support/time_with_zone.rb
index 3155c0ab0f..4a0ed356b1 100644
--- a/activesupport/lib/active_support/time_with_zone.rb
+++ b/activesupport/lib/active_support/time_with_zone.rb
@@ -262,7 +262,7 @@ module ActiveSupport
       # 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.to_f - other.to_f
+        to_time - other.to_time
       elsif duration_of_variable_length?(other)
         method_missing(:-, other)
       else
diff --git a/activesupport/test/core_ext/time_with_zone_test.rb b/activesupport/test/core_ext/time_with_zone_test.rb
index 7fe4d4a6b2..6d779bf3d5 100644
--- a/activesupport/test/core_ext/time_with_zone_test.rb
+++ b/activesupport/test/core_ext/time_with_zone_test.rb
@@ -246,16 +246,31 @@ class TimeWithZoneTest < ActiveSupport::TestCase
     assert_equal  86_400.0,  ActiveSupport::TimeWithZone.new( Time.utc(2000, 1, 2), ActiveSupport::TimeZone['Hawaii'] ) - Time.utc(2000, 1, 1)
   end
 
+  def test_minus_with_time_precision
+    assert_equal  86_399.999999998,  ActiveSupport::TimeWithZone.new( Time.utc(2000, 1, 2, 23, 59, 59, Rational(999999999, 1000)), ActiveSupport::TimeZone['UTC'] ) - Time.utc(2000, 1, 2, 0, 0, 0, Rational(1, 1000))
+    assert_equal  86_399.999999998,  ActiveSupport::TimeWithZone.new( Time.utc(2000, 1, 2, 23, 59, 59, Rational(999999999, 1000)), ActiveSupport::TimeZone['Hawaii'] ) - Time.utc(2000, 1, 2, 0, 0, 0, Rational(1, 1000))
+  end
+
   def test_minus_with_time_with_zone
     twz1 = ActiveSupport::TimeWithZone.new( Time.utc(2000, 1, 1), ActiveSupport::TimeZone['UTC'] )
     twz2 = ActiveSupport::TimeWithZone.new( Time.utc(2000, 1, 2), ActiveSupport::TimeZone['UTC'] )
     assert_equal  86_400.0,  twz2 - twz1
   end
 
+  def test_minus_with_time_with_zone_precision
+    twz1 = ActiveSupport::TimeWithZone.new( Time.utc(2000, 1, 1, 0, 0, 0, Rational(1, 1000)), ActiveSupport::TimeZone['UTC'] )
+    twz2 = ActiveSupport::TimeWithZone.new( Time.utc(2000, 1, 1, 23, 59, 59, Rational(999999999, 1000)), ActiveSupport::TimeZone['UTC'] )
+    assert_equal  86_399.999999998,  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_datetime_precision
+    assert_equal  86_399.999999999,  ActiveSupport::TimeWithZone.new( Time.utc(2000, 1, 1, 23, 59, 59, Rational(999999999, 1000)), 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)