Getting elapsed time with vanilla Javascript based on milliseconds, not using Date methods.

getAge.js

function getAge(birthDate, dateToCompare) {
  const birth = new Date(birthDate);
  const dateToCheck = dateToCompare ? new Date(dateToCompare) : new Date();
  const [initialYear, finalYear] = [birth.getFullYear(), dateToCheck.getFullYear()];

  // getting milliseconds passed from birthdate to the date to check
  const milliseconds = dateToCheck.valueOf() - birth.valueOf();
  // returning age by transforming milliseconds => minutes => hours => days => years
  const age = milliseconds / (1000 * 60 * 60 * 24 * averageDaysPerYear(initialYear, finalYear));
  return parseInt(age);
}

function leapYearsCounter(initialYear, finalYear) {
  let count = 0;
  for (let year = initialYear; year <= finalYear; year++) {
    if (!(year % 4)) count++;
    else if (!(year % 100) && !(year % 400)) count++;
  }
  return count;
}

function averageDaysPerYear(initialYear, finalYear) {
  const leapYearsCount = leapYearsCounter(initialYear, finalYear);
  const years = finalYear - initialYear;
  const average = ((years - leapYearsCount) * 365 + leapYearsCount * 366) / years;
  return average;
}

const age = getAge('1985.11.03', '2020.11.02');

console.log(age); // 34