timeOffset.js

async function timeOffset(url) {
  const t1 = performance.now() + performance.timeOrigin;
  const t2 = await (await fetch(url)).text();
  const t3 = t2;
  const t4 = performance.now() + performance.timeOrigin;
  return ((t2 - t1) + (t3 - t4)) / 2;
}