class Clock extends React.Component {
    constructor(props) {
        super(props);
        let fixDate = new Date("December 8,2019 00:00:00"); 
        let currDate = new Date();//"December 4,2019 00:00:00");
        this.state = { fixDate,currDate };
    }
    componentDidMount() {
        this.timerID = setInterval(() => this.tick(),1000);
    }
    componentWillUnmount() {
        clearInterval(this.timerID);
    }
    tick() {
        this.setState({
            currDate: new Date()//"December 4,2019 00:00:00")
        });
    }
    render() {
        const { fixDate,currDate } = this.state;
     	const diff = fixDate - currDate;
      	//console.log(diff);
        const days = Math.abs(Math.floor(diff / (24 * 60 * 60 * 1000)));
        const totalhours = Math.floor(diff / (60 * 60 * 1000));
        const plusminus = totalhours % 24;
        const hours = Math.abs(totalhours % 24);
        const mins = Math.floor((diff - (totalhours * 60 * 60 * 1000)) / (60 * 1000));
        const secs = Math.floor((diff - (totalhours * 60 * 60 * 1000) - (mins * 60 * 1000)) / 1000);
        if (plusminus < 0) {
            return (
                <div>
                    <h2>Past {days} Days {hours} Hours {mins} Mins {secs} Secs</h2>
                </div>
            );
        } else {
            return (
                <div>
                    <h2>{days} Days {hours} Hours {mins} Mins {secs} Secs to go</h2>
                </div>
            );
        }
    }
}

ReactDOM.render(
  <Clock></Clock>,document.getElementById('container')
);

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>

<div id="container">
</div>