Optimize JS date implementation for smi-only arrays

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 276 matching lines @@
       + TO_INTEGER(ms);
 }
 
 
 // ECMA 262 - 15.9.1.12
 function TimeInYear(year) {
   return DaysInYear(year) * msPerDay;
 }
 
 
-var ymd_from_time_cache = [$NaN, $NaN, $NaN];
-var ymd_from_time_cached_time = $NaN;
+var ymd_from_time_cache = [1970, 0, 1];
+var ymd_from_time_cached_time = 0;
 
 function YearFromTime(t) {
   if (t !== ymd_from_time_cached_time) {
     if (!$isFinite(t)) {
       return $NaN;
     }
 
     %DateYMDFromTime(t, ymd_from_time_cache);
     ymd_from_time_cached_time = t
   }
@@ skipping 797 matching lines @@
     "toGMTString", DateToGMTString,
     "toUTCString", DateToUTCString,
     "getYear", DateGetYear,
     "setYear", DateSetYear,
     "toISOString", DateToISOString,
     "toJSON", DateToJSON
   ));
 }
 
 SetUpDate();