The obvious answer was using performance.timing, as described here, but it is deprecated.

The new interface seems to give only durations, but I can not see any reference of timestamp.

Example (from here):

performance.getEntriesByType('navigation').forEach((navigation) => 
{
  console.dir(navigation);
});

gives

{
    "name": "https://developer.mozilla.org/en-US/docs/Web/Performance/Navigation_and_resource_timings",
    "entryType": "navigation",
    "startTime": 0,
    "duration": 381.90000009536743,
    "initiatorType": "navigation",
    "nextHopProtocol": "h2",
    "workerStart": 0,
    "redirectStart": 0,
    "redirectEnd": 0,
    "fetchStart": 9,
    "domainLookupStart": 9,
    "domainLookupEnd": 9,
    "connectStart": 9,
    "connectEnd": 9,
    "secureConnectionStart": 9,
    "requestStart": 15.400000095367432,
    "responseStart": 49.40000009536743,
    "responseEnd": 52,
    "transferSize": 16819,
    "encodedBodySize": 16519,
    "decodedBodySize": 104257,
    "serverTiming": [],
    "workerTiming": [],
    "unloadEventStart": 0,
    "unloadEventEnd": 0,
    "domInteractive": 271.90000009536743,
    "domContentLoadedEventStart": 324.2000000476837,
    "domContentLoadedEventEnd": 324.2000000476837,
    "domComplete": 381.7000000476837,
    "loadEventStart": 381.90000009536743,
    "loadEventEnd": 381.90000009536743,
    "type": "navigate",
    "redirectCount": 0
}

Besides the documentation or implementation is way more complex. How do we get now the timestamp of the page loaded (or of startTime) ?