Fun React component for animating a changing number

SpinCounter.js

import React from 'react';

/**
 * @param {number} t Number between 0 and 1 indicating the progress of the ease.
 * @param {number} startValue
 * @param {number} endValue
 */
function easeOut(t, startValue, endValue) {
    const c = endValue - startValue;
    const p = t - 1;
    return (-c * ((p * p * p * p) - 1)) + startValue;
}

export default class SpinCounter extends React.Component {
    static propTypes = {
        value: React.PropTypes.number.isRequired,
        duration: React.PropTypes.number,
        easing: React.PropTypes.func,
    };

    constructor(props) {
        super(props);
        this.state = {
            lastValue: props.value,
            displayValue: props.value,
            startTime: +new Date(),
        };
    }

    componentDidMount() {
        setTimeout(() => this.setState({
            lastValue: this.props.value,
            startTime: +new Date(),
            displayValue: this.props.value,
        }), 0);
        this.animInterval = setInterval(() => this.updateDisplayValue(), 1000 / 32);
    }

    componentWillReceiveProps(newProps) {
        const now = +new Date();

        this.setState({
            lastValue: this.state.displayValue,
            startTime: now,
        });

        this.updateDisplayValue(newProps, now);
    }

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

    updateDisplayValue(inprops, instartTime) {
        const now = +new Date();
        const props = inprops || this.props;
        const startTime = instartTime || this.state.startTime;
        const duration = this.props.duration || 1000;
        const ease = this.props.easing || easeOut;
        const t = Math.max(0, Math.min(1, (now - startTime) / duration));
        if (Math.abs(this.state.displayValue - props.value) < 1) {
            this.setState({ displayValue: this.props.value });
            return;
        }
        const displayValue = ease(
            t,
            this.state.lastValue,
            props.value
        );
        this.setState({ displayValue });
    }


    render() {
        return (<div className="counter">
        {Math.round(this.state.displayValue)}
        </div>);
    }
}