diff --git a/lib/mongo_mapper/extensions/time.rb b/lib/mongo_mapper/extensions/time.rb
index f891ed62c..1cfe8710e 100644
--- a/lib/mongo_mapper/extensions/time.rb
+++ b/lib/mongo_mapper/extensions/time.rb
@@ -8,12 +8,7 @@ def to_mongo(value)
         else
           time_class = ::Time.try(:zone).present? ? ::Time.zone : ::Time
           time = value.is_a?(::Time) ? value : time_class.parse(value.to_s)
-          # for ree (1.8.7) versions
-          if RUBY_VERSION < "1.9"
-            at(Float((time.to_f*1000).to_i)/Float(1000)).utc if time # ensure milliseconds are preserved with to_f (issue #308)            
-          else
-            at(time.to_f).utc.round(3) if time # ensure milliseconds are preserved with to_f (issue #308)            
-          end
+          iso8601(time.utc.iso8601(3)).utc if time # keep time with milliseconds. (Same as BSON time)
         end
       end
 
diff --git a/test/unit/test_time_zones.rb b/test/unit/test_time_zones.rb
index 733e2c620..ec6318061 100644
--- a/test/unit/test_time_zones.rb
+++ b/test/unit/test_time_zones.rb
@@ -11,7 +11,7 @@ class TimeZonesTest < Test::Unit::TestCase
 
     should "preserve milliseconds" do
       doc = @document.new(:created_at => '2011-02-12 16:01:02.543Z')
-      doc.created_at.should be_close(Time.parse('2011-02-12 16:01:02.543Z'), 0.0000001)
+      doc.created_at.should be_close(Time.parse('2011-02-12 16:01:02.543Z'), 0.0001)
     end
 
     should "work without Time.zone" do