Как использовать easytimer.js в моем приложении React?

Вот ваше решение

Код Timer.js:

import React, { Component } from "react";
import EasyTimer from "easytimer";

class App extends Component {
    constructor() {
        super();

        this.state = {
            timer: new EasyTimer(),
            timeValues: ""
        };

        this.tick = this.tick.bind(this);
    }

    componentDidMount() {
        let { timer } = this.state;
        timer.start();
        timer.addEventListener("secondsUpdated", this.tick);
    }

    tick(e) {
        let { timer } = this.state;
        const timeValues = timer.getTimeValues().toString();
        this.setState({ timeValues: timeValues });
    }

    render() {
        return <div className = "App">{this.state.timeValues}</div>;
    }
}

export default App;