Add ES specification reference to DateNow.

src/date.js

 // Copyright 2006-2008 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 284 matching lines @@
   ms = argc > 6 ? ToNumber(ms) : 0;
   year = (!NUMBER_IS_NAN(year) &&
           0 <= TO_INTEGER(year) &&
           TO_INTEGER(year) <= 99) ? 1900 + TO_INTEGER(year) : year;
   var day = MakeDay(year, month, date);
   var time = MakeTime(hours, minutes, seconds, ms);
   return TimeClip(MakeDate(day, time));
 }
 
 
-// Mozilla-specific extension. Returns the number of milliseconds
-// elapsed since 1 January 1970 00:00:00 UTC.
+// ECMA 262 - 15.9.4.4
 function DateNow() {
   return %DateCurrentTime();
 }
 
 
 // ECMA 262 - 15.9.5.2
 function DateToString() {
   var t = UTC_DATE_VALUE(this)
   if (NUMBER_IS_NAN(t)) return kInvalidDate;
   var time_zone_string = LocalTimezoneString(this)
@@ skipping 503 matching lines @@
     "toGMTString", DateToGMTString,
     "toUTCString", DateToUTCString,
     "getYear", DateGetYear,
     "setYear", DateSetYear,
     "toISOString", DateToISOString,
     "toJSON", DateToJSON
   ));
 }
 
 SetUpDate();