fix of #799

Author: AJIXuMuK

src/controls/dateTimePicker/DateTimePicker.tsx

@@ -69,7 +69,7 @@ export class DateTimePicker extends React.Component<IDateTimePickerProps, IDateT
    * Called before the component receives new props, used for matching state with new props.
    */
   public componentWillReceiveProps(nextProps: IDateTimePickerProps): void {
-    if (nextProps.value != null && !isEqual(nextProps.value, this.props.value)) {
+    if (!isEqual(nextProps.value, this.props.value)) {
       const { day, hours, minutes, seconds } = DateTimePicker.getDateComponents(nextProps.value, this.props.dateConvention);
       this.setState({ day, hours, minutes, seconds });
     }
@@ -134,9 +134,9 @@ export class DateTimePicker extends React.Component<IDateTimePickerProps, IDateT
           }
         }
         else if (hours === 12) {
-            //am - if hours == 12, set hours to 0 here
-            hours = 0;
-          }
+          //am - if hours == 12, set hours to 0 here
+          hours = 0;
+        }
       }
 
       if (hours > 23) {

src/webparts/controlsTest/components/ControlsTest.tsx

@@ -1121,13 +1121,10 @@ export default class ControlsTest extends React.Component<IControlsTestProps, IC
           showSeconds={true}
           timeDisplayControlType={TimeDisplayControlType.Dropdown}
         />
-        <PrimaryButton text={'Change Date'} onClick={() => {
-          const date = this.state.dateTimeValue || new Date();
-          date.setMinutes(50);
-          date.setHours(11);
-          date.setSeconds(12);
+        <PrimaryButton text={'Clear Date'} onClick={() => {
+
           this.setState({
-            dateTimeValue: date
+            dateTimeValue: undefined
           });
         }} />