fix(input[date]): do not use $isEmpty to check the model validity

Author: petebacondarwin

src/ng/directive/input.js

@@ -1145,10 +1145,10 @@ function createDateInputType(type, regexp, parseDate, format) {
     });
 
     ctrl.$formatters.push(function(value) {
-      if (!ctrl.$isEmpty(value)) {
-        if (!isDate(value)) {
-          throw $ngModelMinErr('datefmt', 'Expected `{0}` to be a date', value);
-        }
+      if (value && !isDate(value)) {
+        throw $ngModelMinErr('datefmt', 'Expected `{0}` to be a date', value);
+      }
+      if (isValidDate(value)) {
         previousDate = value;
         if (previousDate && timezone === 'UTC') {
           var timezoneOffset = 60000 * previousDate.getTimezoneOffset();
@@ -1157,14 +1157,14 @@ function createDateInputType(type, regexp, parseDate, format) {
         return $filter('date')(value, format, timezone);
       } else {
         previousDate = null;
+        return '';
       }
-      return '';
     });
 
     if (isDefined(attr.min) || attr.ngMin) {
       var minVal;
       ctrl.$validators.min = function(value) {
-        return ctrl.$isEmpty(value) || isUndefined(minVal) || parseDate(value) >= minVal;
+        return !isValidDate(value) || isUndefined(minVal) || parseDate(value) >= minVal;
       };
       attr.$observe('min', function(val) {
         minVal = parseObservedDateValue(val);
@@ -1175,18 +1175,18 @@ function createDateInputType(type, regexp, parseDate, format) {
     if (isDefined(attr.max) || attr.ngMax) {
       var maxVal;
       ctrl.$validators.max = function(value) {
-        return ctrl.$isEmpty(value) || isUndefined(maxVal) || parseDate(value) <= maxVal;
+        return !isValidDate(value) || isUndefined(maxVal) || parseDate(value) <= maxVal;
       };
       attr.$observe('max', function(val) {
         maxVal = parseObservedDateValue(val);
         ctrl.$validate();
       });
     }
-    // Override the standard $isEmpty to detect invalid dates as well
-    ctrl.$isEmpty = function(value) {
+
+    function isValidDate(value) {
       // Invalid Date: getTime() returns NaN
-      return !value || (value.getTime && value.getTime() !== value.getTime());
-    };
+      return value && !(value.getTime && value.getTime() !== value.getTime());
+    }
 
     function parseObservedDateValue(val) {
       return isDefined(val) ? (isDate(val) ? val : parseDate(val)) : undefined;