fix(datepicker): reformat valid values on blur

src/lib/datepicker/datepicker-input.ts

@@ -87,7 +87,7 @@ export class MatDatepickerInputEvent<D> {
     '[disabled]': 'disabled',
     '(input)': '_onInput($event.target.value)',
     '(change)': '_onChange()',
-    '(blur)': '_onTouched()',
+    '(blur)': '_onBlur()',
     '(keydown)': '_onKeydown($event)',
   },
   exportAs: 'matDatepickerInput',
@@ -123,10 +123,10 @@ export class MatDatepickerInput<D> implements AfterContentInit, ControlValueAcce
     value = this._dateAdapter.deserialize(value);
     this._lastValueValid = !value || this._dateAdapter.isValid(value);
     value = this._getValidDateOrNull(value);
-    let oldDate = this.value;
+    const oldDate = this.value;
     this._value = value;
-    this._elementRef.nativeElement.value =
-        value ? this._dateAdapter.format(value, this._dateFormats.display.dateInput) : '';
+    this._formatValue(value);
+
     if (!this._dateAdapter.sameDate(oldDate, value)) {
       this._valueChange.emit(value);
     }
@@ -343,6 +343,22 @@ export class MatDatepickerInput<D> implements AfterContentInit, ControlValueAcce
     return this._formField ? this._formField.color : undefined;
   }
 
+  /** Handles blur events on the input. */
+  _onBlur() {
+    // Reformat the input only if we have a valid value.
+    if (this.value) {
+      this._formatValue(this.value);
+    }
+
+    this._onTouched();
+  }
+
+  /** Formats a value and sets it on the input element. */
+  private _formatValue(value: D | null) {
+    this._elementRef.nativeElement.value =
+        value ? this._dateAdapter.format(value, this._dateFormats.display.dateInput) : '';
+  }
+
   /**
    * @param obj The object to check.
    * @returns The given object if it is both a date instance and valid, otherwise null.

src/lib/datepicker/datepicker.spec.ts

@@ -663,6 +663,38 @@ describe('MatDatepicker', () => {
         expect(inputEl.classList).toContain('ng-touched');
       });
 
+      it('should reformat the input value on blur', () => {
+        if (SUPPORTS_INTL) {
+          // Skip this test if the internationalization API is not supported in the current
+          // browser. Browsers like Safari 9 do not support the "Intl" API.
+          return;
+        }
+
+        const inputEl = fixture.debugElement.query(By.css('input')).nativeElement;
+
+        inputEl.value = '2001-01-01';
+        dispatchFakeEvent(inputEl, 'input');
+        fixture.detectChanges();
+
+        dispatchFakeEvent(inputEl, 'blur');
+        fixture.detectChanges();
+
+        expect(inputEl.value).toBe('1/1/2001');
+      });
+
+      it('should not reformat invalid dates on blur', () => {
+        const inputEl = fixture.debugElement.query(By.css('input')).nativeElement;
+
+        inputEl.value = 'very-valid-date';
+        dispatchFakeEvent(inputEl, 'input');
+        fixture.detectChanges();
+
+        dispatchFakeEvent(inputEl, 'blur');
+        fixture.detectChanges();
+
+        expect(inputEl.value).toBe('very-valid-date');
+      });
+
       it('should mark input touched on calendar selection', fakeAsync(() => {
         let inputEl = fixture.debugElement.query(By.css('input')).nativeElement;