Updated VM and JS versions of Date to allow underflow and overflow.

runtime/lib/date.dart

Index: runtime/lib/date.dart
diff --git a/runtime/lib/date.dart b/runtime/lib/date.dart
index 71c079fa7b6e40c1ed5d2b10b0efce6c3a5cc51a..59bdb23a915b2dc54a304081ab118aababb97c3d 100644
--- a/runtime/lib/date.dart
+++ b/runtime/lib/date.dart
@@ -7,7 +7,7 @@
 class DateImplementation implements Date {
   static final int _MAX_MILLISECONDS_SINCE_EPOCH = 8640000000000000;
 
-  DateImplementation(int years,
+  DateImplementation(int year,

[Lasse Reichstein Nielsen, 2012/08/24 08:39:02]

Please remove "Implementation". It's never a good name. Ever. I mean it.
Find something that distinguishes this from any other Date instance. Call it
VMDate for all I care. Just. Not. "Implementation"!
Or, preferably, combine it with Date.

                      [int month = 1,
                       int day = 1,
                       int hour = 0,
@@ -17,7 +17,7 @@ class DateImplementation implements Date {
                       bool isUtc = false])
       : this.isUtc = isUtc,
         this.millisecondsSinceEpoch = _brokenDownDateToMillisecondsSinceEpoch(
-            years, month, day, hour, minute, second, millisecond, isUtc) {
+            year, month, day, hour, minute, second, millisecond, isUtc) {
     if (millisecondsSinceEpoch === null) throw new IllegalArgumentException();
     if (isUtc === null) throw new IllegalArgumentException();
   }
@@ -54,7 +54,7 @@ class DateImplementation implements Date {
         return Math.parseDouble(matched);
       }
 
-      int years = Math.parseInt(match[1]);
+      int year = Math.parseInt(match[1]);
       int month = Math.parseInt(match[2]);
       int day = Math.parseInt(match[3]);
       int hour = parseIntOrZero(match[4]);
@@ -69,7 +69,7 @@ class DateImplementation implements Date {
       // TODO(floitsch): we should not need to test against the empty string.
       bool isUtc = (match[8] !== null) && (match[8] != "");
       int millisecondsSinceEpoch = _brokenDownDateToMillisecondsSinceEpoch(
-          years, month, day, hour, minute, second, millisecond, isUtc);
+          year, month, day, hour, minute, second, millisecond, isUtc);
       if (millisecondsSinceEpoch === null) {
         throw new IllegalArgumentException(formattedString);
       }
@@ -340,23 +340,26 @@ class DateImplementation implements Date {
   }
 
   static _brokenDownDateToMillisecondsSinceEpoch(
-      int years, int month, int day,
+      int year, int month, int day,
       int hour, int minute, int second, int millisecond,
       bool isUtc) {
-    if ((month < 1) || (month > 12)) return null;
-    if ((day < 1) || (day > 31)) return null;
-    // Leap seconds can lead to hour == 24.
-    if ((hour < 0) || (hour > 24)) return null;
-    if ((hour == 24) && ((minute != 0) || (second != 0))) return null;
-    if ((minute < 0) || (minute > 59)) return null;
-    if ((second < 0) || (second > 59)) return null;
-    if ((millisecond < 0) || (millisecond > 999)) return null;
+    // JavaScript month is zero-based while Dart is one-based. This should
+    // simplify some calculations.

[Lasse Reichstein Nielsen, 2012/08/24 08:39:02]

Why reference JavaScript here? This is VM code, right?
It's simply simpler to work with zero based months and days.

[floitsch, 2012/08/24 22:32:49]

My fault.

+    --month;
+    // Deal with under and overflow.
+    year += (month / 12).floor().toInt();
+    month = month % 12;
+    int clampedMonth = month.remainder(12);

[Lasse Reichstein Nielsen, 2012/08/24 08:39:02]

How can this be necessary when %12 above gives a value in the range 0..11?

[floitsch, 2012/08/24 22:32:49]

Agreed. Either use 
year += month ~/ 12;
clampedMonth = month.remainder(12);
if (...)

or use
year += (month / 12).floor().toInt();
month = month % 12;

+    if (clampedMonth < 0) {
+      --year;
+      clampedMonth += 12;
+    }
 
     // First compute the seconds in UTC, independent of the [isUtc] flag. If
     // necessary we will add the time-zone offset later on.
     int days = day - 1;
-    days += _DAYS_UNTIL_MONTH[_isLeapYear(years) ? 1 : 0][month - 1];
-    days += _dayFromYear(years);
+    days += _DAYS_UNTIL_MONTH[_isLeapYear(year) ? 1 : 0][clampedMonth];
+    days += _dayFromYear(year);
     int millisecondsSinceEpoch = days * Duration.MILLISECONDS_PER_DAY +
         hour * Duration.MILLISECONDS_PER_HOUR +
         minute * Duration.MILLISECONDS_PER_MINUTE+