Add safe handling of NaN to Posix platform-dependent time functions.

src/date-delay.js

Index: src/date-delay.js
===================================================================
--- src/date-delay.js	(revision 2614)
+++ src/date-delay.js	(working copy)
@@ -156,8 +156,7 @@
 
 // NOTE: The implementation relies on the fact that no time zones have
 // more than one daylight savings offset change per month.
-// This function must never be called with the argument NaN.
-// All uses of it are guarded so this does not happen.
+// If this function is called with NaN it returns NaN.
 function DaylightSavingsOffset(t) {
   // Load the cache object from the builtins object.
   var cache = DST_offset_cache;
@@ -652,12 +651,13 @@
 
 
 function LocalTimezoneString(time) {
-  // time is not NaN because of checks in calling functions.
-  var timezoneOffset = (local_time_offset + DaylightSavingsOffset(time)) / msPerMinute;
+  var timezoneOffset =
+      (local_time_offset + DaylightSavingsOffset(time)) / msPerMinute;
   var sign = (timezoneOffset >= 0) ? 1 : -1;
   var hours = FLOOR((sign * timezoneOffset)/60);
   var min   = FLOOR((sign * timezoneOffset)%60);
-  var gmt = ' GMT' + ((sign == 1) ? '+' : '-') + TwoDigitString(hours) + TwoDigitString(min);
+  var gmt = ' GMT' + ((sign == 1) ? '+' : '-') +
+      TwoDigitString(hours) + TwoDigitString(min);
   return gmt + ' (' +  LocalTimezone(time) + ')';
 }