RUBY-1702 Skip creating Time instance in Date serialization (mongodb#118

)

* Skip creating Time instance in Date serialization

* Fix spelling and mark constants private

* Move constants to BSON namespace

lib/bson/date.rb

@@ -16,6 +16,16 @@
 
 module BSON
 
+  # Julian day of Date 1970-01-01 - UNIX timestamp reference.
+  #
+  # @api private
+  DATE_REFERENCE = ::Date.new(1970, 1, 1).jd
+
+  # Number of miliseconds in a day.
+  #
+  # @api private
+  MILLISECONDS_IN_DAY = 60 * 60 * 24 * 1_000
+
   # Injects behaviour for encoding date values to raw bytes as specified by
   # the BSON spec for time.
   #
@@ -35,7 +45,7 @@ module Date
     #
     # @since 2.1.0
     def to_bson(buffer = ByteBuffer.new, validating_keys = Config.validating_keys?)
-      ::Time.utc(year, month, day).to_bson(buffer)
+      buffer.put_int64((jd - DATE_REFERENCE) * MILLISECONDS_IN_DAY)
     end
 
     # Get the BSON type for the date.