testdate.js

let seedDate = Date.now();
let sampleDateStrings = [];
let sampleDateValues = [];

const TOTAL_SAMPLE = 1000000;

for (let i = 0; i < TOTAL_SAMPLE; ++i) {
  sampleDateStrings.push((new Date(seedDate + i)).toISOString());
  sampleDateValues.push(seedDate + i);
}

function testDate(samples) {
  let start = Date.now();

  for (let i = 0; i < samples.length; ++i) {
    let d = new Date(samples[i]);
  }

  let end = Date.now();

  return end - start;
}

console.log('Date string: %d', testDate(sampleDateStrings));
console.log('Date value: %d', testDate(sampleDateValues));