date-diff.mjs

const dateDiff = (startDate, endDate) => {
  const startUTC = dateToUTC(startDate);
  const endUTC = dateToUTC(endDate);

  const diffMilliseconds = endUTC - startUTC;
  const millisecondsPerDay = 1000 * 60 * 60 * 24;
  const diffDays = diffMilliseconds / millisecondsPerDay;

  const [years, yearsRem] = floordivmod(diffDays, 365);
  const [months, monthsRem] = floordivmod(yearsRem, 30);
  const [days, daysRem] = floormod(monthsRem);
  const [hours, hoursRem] = floormod(daysRem * 24);
  const [minutes, minutesRem] = floormod(hoursRem * 60);
  const [seconds, secondsRem] = floormod(minutesRem * 60);
  const [milliseconds] = floormod(secondsRem * 1000);

  return { years, months, days, hours, minutes, seconds, milliseconds };
};

const dateToUTC = (date) =>
  Date.UTC(
    date.getFullYear(),
    date.getMonth(),
    date.getDate(),
    date.getHours(),
    date.getMinutes(),
    date.getSeconds(),
    date.getMilliseconds()
  );

const floordivmod = (n, m) => {
  let q = Math.floor(n / m);
  let r = n % m;
  return [q, r];
};

const floormod = (n) => {
  if (n === 0) {
    return [0, 0];
  }
  let q = Math.floor(n);
  let r = n % q;
  return [q, r];
};

const main = () => {
  const args = process.argv.slice(2);
  let startDate, endDate;

  if (args.length === 1) {
    startDate = new Date(args[0]);
    endDate = new Date();
  } else if (args.length === 2) {
    startDate = new Date(args[0]);
    endDate = new Date(args[1]);
  }

  if (startDate && endDate) {
    const diff = dateDiff(startDate, endDate);
    console.log(diff);
  }
};

main();