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..c097374b10dbfdda62f4d3e232f17301c17ccd95 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,
                      [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);
       }
@@ -246,8 +246,9 @@ class DateImplementation implements Date {
   /** The first list contains the days until each month in non-leap years. The
     * second list contains the days in leap years. */
   static final List<List<int>> _DAYS_UNTIL_MONTH =
-      const [const [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334],
-             const [0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335]];
+      const [
+          const [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365],
+          const [0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366]];
 
   // Returns the UTC year, month and day for the corresponding
   // [millisecondsSinceEpoch].
@@ -340,23 +341,31 @@ 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;
+    // Deal with under and overflow.
+    second += (millisecond / 1000).floor().toInt();

[floitsch, 2012/08/14 08:31:38]

You shouldn't need to look at the hours, minutes, seconds and milliseconds. We
don't care if they over or underflow since we end up adding them anyways.
Lines 347 to 355 can be removed.

[dominich, 2012/08/14 16:49:20]

Of course - good point :)

+    millisecond = millisecond % 1000;
+    minute += (second / 60).floor().toInt();
+    second = second % 60;
+    hour += (minute / 60).floor().toInt();
+    minute = minute % 60;
+    day += (hour / 24).floor().toInt();
+    hour = hour % 24;
+    year += ((month - 1) / 12).floor().toInt();
+    month = (month - 1) % 12 + 1;

[floitsch, 2012/08/14 08:31:38]

Instead of dealing with +/- 1 all over the place do it once in the beginning
with a comment. For example:
// Dart's month is 1-indexed. Remove 1 to make it zero-based to simplify
computations.
month--;

[dominich, 2012/08/14 16:49:20]

Done.

+    int clampedMonth = (month - 1).remainder(12) + 1;
+    if (clampedMonth < 1) {
+      --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 - 1];
+    days += _dayFromYear(year);
     int millisecondsSinceEpoch = days * Duration.MILLISECONDS_PER_DAY +
         hour * Duration.MILLISECONDS_PER_HOUR +
         minute * Duration.MILLISECONDS_PER_MINUTE+