How can I convert performance.now() output to a UTC String?

In our code, we were previously calculating the difference between to events like so:

var beginTime = new Date();
// Do stuff
var endTime = new Date();
var duration = endTime.getTime() - beginTime.getTime();

console.log("Job began at " + beginTime.toUTCString()
            + " and took " + duration + " milliseconds.");

This results in a human-readable string:

Job began at Thu Sep 28 2017 11:17:33 GMT-0500 (Central Daylight Time) and took 7000 milliseconds.

We've decided to switch to High Resolution Time by using the more reliable performance.now(). However, we still want to be able have a human-readable UTC time string included.

Initially, we tried this:

var beginTime = performance.now();
// Do stuff
var endTime = performance.now();
var duration = endTime - beginTime;

console.log("Job began at " + new Date(beginTime).toUTCString() 
            + " and took " + duration + " seconds.");

We are finding that the duration is accurate, but new Date(performance.now()) results in an inaccurate UTC value (at the time of this writing, it provides a date nearly 50 years in the past).

Job began at Wed Dec 31 1969 20:10:46 GMT-0600 (Central Standard Time) and took 7000 milliseconds.

Is there a better way to convert the output of performance.now() to an accurate UTC string? It doesn't have to be the exact same format as new Date().toUTCString(), but it should be human-readable.

It can be done like this:

const t0 = performance.now();

// measured job here

const t1     = performance.now(),
      t0Date = new Date(performance.timing.navigationStart + t0).toUTCString();

console.log(`Job began at ${t0Date} and took ${t1 - t0} milliseconds.`);

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Note however that after performance.now() MDN page (emphasis mine):

(…) performance.timing.navigationStart + performance.now() will be approximately equal to Date.now().

For me, it's within one second of actual time start.