[performance.now()](<https://developer.mozilla.org/en-US/docs/Web/API/Performance/now>)
returns a precise timestamp: The number of milliseconds, including microseconds, since the current web page started to load.
More generally, it returns the time elapsed since the [performanceTiming.navigationStart](<https://developer.mozilla.org/en-US/docs/Web/API/PerformanceTiming/navigationStart>)
event.
t = performance.now();
For example, in a web browser’s main context, performance.now()
returns 6288.319
if the web page began to load 6288 milliseconds and 319 microseconds ago.