Persist program state after refresh

Question

I'm implementing countdown using Vanilla JS and I'd like to know how do I ensure that it doesn't reset each time I refresh the page.

Am I missing something here? As for now, it will reset to 30 days each time I reload the page.

const countDownClock = (number = 100, format = 'seconds') => {

  const daysElement = document.querySelector('.days');
  const hoursElement = document.querySelector('.hours');
  const minutesElement = document.querySelector('.minutes');
  const secondsElement = document.querySelector('.seconds');
  let countdown;
  convertFormat(format);

  function convertFormat(format) {
    switch (format) {
      case 'seconds':
        return timer(number);
      case 'minutes':
        return timer(number * 60);
      case 'hours':
        return timer(number * 60 * 60);
      case 'days':
        return timer(number * 60 * 60 * 24);
    }
  }

  function timer(seconds) {
    const now = Date.now();
    const then = now + seconds * 1000;

    countdown = setInterval(() => {
      const secondsLeft = Math.round((then - Date.now()) / 1000);

      if (secondsLeft <= 0) {
        clearInterval(countdown);
        return;
      };

      displayTimeLeft(secondsLeft);

    }, 1000);
  }

  function displayTimeLeft(seconds) {
    daysElement.textContent = Math.floor(seconds / 86400);
    hoursElement.textContent = Math.floor((seconds % 86400) / 3600);
    minutesElement.textContent = Math.floor((seconds % 86400) % 3600 / 60);
    secondsElement.textContent = seconds % 60 < 10 ? `0${seconds % 60}` : seconds % 60;
  }
}

countDownClock(30, 'days');