React

HTML

<div id="app"></div>

CSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

#app {
  background: #fff;
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}

li {
  margin: 8px 0;
}

h2 {
  font-weight: bold;
  margin-bottom: 15px;
}

.done {
  color: rgba(0, 0, 0, 0.3);
  text-decoration: line-through;
}

input {
  margin-right: 5px;
}

React

class TodoApp extends React.Component {
  constructor(props) {
    super(props)
    this.state = {
    	datetime: new Date()
    }
  }
  
  componentDidMount() {
  	setInterval(() => {
    	this.setState({datetime: new Date()})
    }, 1000);  	
  }
  
  getDateTime() {
      let now     = new Date(); 
      let year    = now.getFullYear();
      let month   = now.getMonth()+1; 
      let day     = now.getDate();
      let hour    = now.getHours();
      let minute  = now.getMinutes();
      let second  = now.getSeconds(); 
      if(month.toString().length === 1) {
           month = '0'+month;
      }
      if(day.toString().length === 1) {
           day = '0'+day;
      }   
      if(hour.toString().length === 1) {
           hour = '0'+hour;
      }
      if(minute.toString().length === 1) {
           minute = '0'+minute;
      }
      if(second.toString().length === 1) {
           second = '0'+second;
      }   
      var dateTime = hour+':'+minute+':'+second;   
       return dateTime;
  }
  
  render() {
    return (
      <div>
        <h2>The time is {this.getDateTime()}</h2>
      </div>
    )
  }
}

ReactDOM.render(<TodoApp />, document.querySelector("#app"))