diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 0f9399f4d65c2..1863eec947121 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Add `DateTime#usec` and `DateTime#nsec` so that `ActiveSupport::TimeWithZone` keeps
+    sub-second resolution when wrapping a `DateTime` value.
+    Fixes #10855
+
+    *Andrew White*
+
 *   Fix `ActiveSupport::Dependencies::Loadable#load_dependency` calling
     `#blame_file!` on Exceptions that do not have the Blamable mixin
 
diff --git a/activesupport/lib/active_support/core_ext/date_time/conversions.rb b/activesupport/lib/active_support/core_ext/date_time/conversions.rb
index df07917d194a4..c44626aed9a84 100644
--- a/activesupport/lib/active_support/core_ext/date_time/conversions.rb
+++ b/activesupport/lib/active_support/core_ext/date_time/conversions.rb
@@ -80,6 +80,16 @@ def to_i
     seconds_since_unix_epoch.to_i
   end
 
+  # Returns the fraction of a second as microseconds
+  def usec
+    (sec_fraction * 1_000_000).to_i
+  end
+
+  # Returns the fraction of a second as nanoseconds
+  def nsec
+    (sec_fraction * 1_000_000_000).to_i
+  end
+
   private
 
   def offset_in_seconds
diff --git a/activesupport/lib/active_support/time_with_zone.rb b/activesupport/lib/active_support/time_with_zone.rb
index 4a032b0ad0653..95b9b8e5aedfd 100644
--- a/activesupport/lib/active_support/time_with_zone.rb
+++ b/activesupport/lib/active_support/time_with_zone.rb
@@ -292,7 +292,7 @@ def advance(options)
       end
     end
 
-    %w(year mon month day mday wday yday hour min sec to_date).each do |method_name|
+    %w(year mon month day mday wday yday hour min sec usec nsec to_date).each do |method_name|
       class_eval <<-EOV, __FILE__, __LINE__ + 1
         def #{method_name}    # def month
           time.#{method_name} #   time.month
@@ -300,10 +300,6 @@ def #{method_name}    # def month
       EOV
     end
 
-    def usec
-      time.respond_to?(:usec) ? time.usec : 0
-    end
-
     def to_a
       [time.sec, time.min, time.hour, time.day, time.mon, time.year, time.wday, time.yday, dst?, zone]
     end
diff --git a/activesupport/test/core_ext/date_time_ext_test.rb b/activesupport/test/core_ext/date_time_ext_test.rb
index 3e76b8a9b16b3..571344b728ba0 100644
--- a/activesupport/test/core_ext/date_time_ext_test.rb
+++ b/activesupport/test/core_ext/date_time_ext_test.rb
@@ -327,6 +327,16 @@ def test_to_i
     assert_equal 946684800, DateTime.civil(1999,12,31,19,0,0,Rational(-5,24)).to_i
   end
 
+  def test_usec
+    assert_equal 0, DateTime.civil(2000).usec
+    assert_equal 500000, DateTime.civil(2000, 1, 1, 0, 0, Rational(1,2)).usec
+  end
+
+  def test_nsec
+    assert_equal 0, DateTime.civil(2000).nsec
+    assert_equal 500000000, DateTime.civil(2000, 1, 1, 0, 0, Rational(1,2)).nsec
+  end
+
   protected
     def with_env_tz(new_tz = 'US/Eastern')
       old_tz, ENV['TZ'] = ENV['TZ'], new_tz
diff --git a/activesupport/test/core_ext/time_with_zone_test.rb b/activesupport/test/core_ext/time_with_zone_test.rb
index 3ce3297874537..e8eea27d72f4d 100644
--- a/activesupport/test/core_ext/time_with_zone_test.rb
+++ b/activesupport/test/core_ext/time_with_zone_test.rb
@@ -445,6 +445,11 @@ def test_usec_returns_0_when_datetime_is_wrapped
     assert_equal 0, twz.usec
   end
 
+  def test_usec_returns_sec_fraction_when_datetime_is_wrapped
+    twz = ActiveSupport::TimeWithZone.new(DateTime.civil(2000, 1, 1, 0, 0, Rational(1,2)), @time_zone)
+    assert_equal 500000, twz.usec
+  end
+
   def test_utc_to_local_conversion_saves_period_in_instance_variable
     assert_nil @twz.instance_variable_get('@period')
     @twz.time