countdown.js

import React from 'react';
import moment from 'moment';

import styles from './component.pcss';

class Countdown extends React.Component {
    interval = null;

    componentDidMount() {
        this.interval = setInterval(() => this.setState({ time: Date.now() }), 1000);
    }

    componentWillUnmount() {
        clearInterval(this.interval);
    }

    render() {
        const { startedAt } = this.props;
        const now = moment();

        const nowSeconds = now.seconds();
        const startedAtSeconds = startedAt.seconds();
        const dif = nowSeconds - startedAtSeconds;

        const minutes = parseInt(moment.duration(startedAt.diff(now)).asMinutes());
        const seconds = dif > 0 ? 60 - dif : Math.abs(dif);

        return (
            <div className={styles.root}>
                <p>
                    {
                        `${minutes}:${seconds}`
                    }
                </p>
            </div>
        );
    }
}

export default Countdown;