- millisecondsAndMicrosecondsSincePageLoad = performance.now();
- millisecondsSinceYear1970 = Date.now();
- millisecondsSinceYear1970 = (new Date()).getTime();
performance.now() is available in modern web browsers and provides reliable timestamps with sub-millisecond resolution.
(new Date()).getTime() are based on the system time, they often get skewed by a few milliseconds when the system time is automatically synchronized.