cesiumDemo/Source/Core/getTimestamp.js

    /**
     * Gets a timestamp that can be used in measuring the time between events.  Timestamps
     * are expressed in milliseconds, but it is not specified what the milliseconds are
     * measured from.  This function uses performance.now() if it is available, or Date.now()
     * otherwise.
     *
     * @exports getTimestamp
     *
     * @returns {Number} The timestamp in milliseconds since some unspecified reference time.
     */
    var getTimestamp;

    if (typeof performance !== 'undefined' && typeof performance.now === 'function' && isFinite(performance.now())) {
        getTimestamp = function() {
            return performance.now();
        };
    } else {
        getTimestamp = function() {
            return Date.now();
        };
    }
export default getTimestamp;