fix(datepicker): fix input always being required.

src/components/datepicker/datePicker.js

@@ -367,13 +367,15 @@
    * @param {Date=} opt_date Date to check. If not given, defaults to the datepicker's model value.
    */
   DatePickerCtrl.prototype.updateErrorState = function(opt_date) {
-    // Force all dates to midnight in order to ignore the time portion.
-    var date = this.dateUtil.createDateAtMidnight(opt_date || this.date);
+    var date = opt_date || this.date;
 
     // Clear any existing errors to get rid of anything that's no longer relevant.
     this.clearErrorState();
 
     if (this.dateUtil.isValidDate(date)) {
+      // Force all dates to midnight in order to ignore the time portion.
+      date = this.dateUtil.createDateAtMidnight(date);
+
       if (this.dateUtil.isValidDate(this.minDate)) {
         var minDate = this.dateUtil.createDateAtMidnight(this.minDate);
         this.ngModelCtrl.$setValidity('mindate', date >= minDate);

src/components/datepicker/datePicker.spec.js

@@ -189,6 +189,24 @@ describe('md-date-picker', function() {
       expect(controller.ngModelCtrl.$error['mindate']).toBeFalsy();
     });
 
+    it('should not enforce `required` when a min-date is set', function() {
+      pageScope.isRequired = false;
+      pageScope.minDate = new Date(2015, JAN, 1);
+      pageScope.myDate = null;
+      pageScope.$apply();
+
+      expect(controller.ngModelCtrl.$error['mindate']).toBeFalsy();
+    });
+
+    it('should not enforce `required` when a max-date is set', function() {
+      pageScope.isRequired = false;
+      pageScope.maxDate = new Date(2015, JAN, 1);
+      pageScope.myDate = null;
+      pageScope.$apply();
+
+      expect(controller.ngModelCtrl.$error['mindate']).toBeFalsy();
+    });
+
     describe('inside of a form element', function() {
       var formCtrl;