Fix the type of the DateTimePicker's change callback.

src/components/EventEdit.tsx

@@ -218,7 +218,7 @@ class EventEdit extends React.PureComponent {
             <DateTimePicker
               dateOnly={this.state.allDay}
               value={this.state.start}
-              onChange={(date: Date) => this.setState({start: date})}
+              onChange={(date: string) => this.setState({start: date})}
             />
           </div>
 
@@ -226,7 +226,7 @@ class EventEdit extends React.PureComponent {
             <DateTimePicker
               dateOnly={this.state.allDay}
               value={this.state.end}
-              onChange={(date: Date) => this.setState({end: date})}
+              onChange={(date: string) => this.setState({end: date})}
             />
           </div>
 

src/widgets/DateTimePicker.tsx

@@ -9,7 +9,7 @@ class DateTimePicker extends React.PureComponent {
   props: {
     value?: string,
     dateOnly?: boolean,
-    onChange: (date: Date) => void;
+    onChange: (date: string) => void;
   };
 
   constructor(props: any) {