Countdown timer with a weekly reset

coundown.html

<!-- @link: https://stackoverflow.com/questions/35587387/javascript-countdown-clock-every-saturday-at-11am#answer-35588057 -->
    <div class="has-time">
      <div id="clockdiv" class="is-time simply-countdown">
        <div class="simply-section">
          <span class="days simply-amount"></span>
          <div class="smalltext simply-word">Days</div>
        </div>
        <div class="simply-section">
          <span class="hours simply-amount"></span>
          <div class="smalltext simply-word">Hours</div>
        </div>
        <div class="simply-section">
          <span class="minutes simply-amount"></span>
          <div class="smalltext simply-word">Minutes</div>
        </div>
        <div class="simply-section">
          <span class="seconds simply-amount"></span>
          <div class="smalltext simply-word">Seconds</div>
        </div>
      </div>
    </div>

countdown-refined-with-notes.js

/**
 * @link: https://stackoverflow.com/questions/35587387/javascript-countdown-clock-every-saturday-at-11am#answer-35588057
 */
(function() {
  'use strict';
  
  /**
   * Calculate time remaining until a given end Date.
   */
  function getTimeRemaining(endtime) {
    var t = Date.parse(endtime) - Date.parse(new Date());
    var seconds = Math.floor((t / 1000) % 60);
    var minutes = Math.floor((t / 1000 / 60) % 60);
    var hours = Math.floor((t / (1000 * 60 * 60)) % 24);
    var days = Math.floor(t / (1000 * 60 * 60 * 24));
    return {
      total: t,
      days: days,
      hours: hours,
      minutes: minutes,
      seconds: seconds
    };
  }

  /**
   * Set up and start a countdown clock.
   *
   * Countdown target is the upcoming Sunday.
   */
  function initializeClock(id) {
    var clock = document.getElementById(id);
    var daysSpan = clock.querySelector(".days");
    var hoursSpan = clock.querySelector(".hours");
    var minutesSpan = clock.querySelector(".minutes");
    var secondsSpan = clock.querySelector(".seconds");
    
    // Set an initial end time when the page loads.
    var endtime = convertToEST(getNextSunday());

    function updateClock() {
      var t = getTimeRemaining(endtime);

      // When countdown expires, reset the end time.
      if (t.total <= 0) {
        endtime = convertToEST(getNextSunday());
      }

      daysSpan.innerHTML = t.days;
      hoursSpan.innerHTML = ("0" + t.hours).slice(-2);
      minutesSpan.innerHTML = ("0" + t.minutes).slice(-2);
      secondsSpan.innerHTML = ("0" + t.seconds).slice(-2);

    }

    updateClock();
    var timeinterval = setInterval(updateClock, 1000);
  }

  /**
   * Find the Date of the upcoming Sunday at 9AM UTC.
   */
  function getNextSunday() {
    var now = new Date();
    var nextSunday = new Date();
    nextSunday.setDate(now.getDate() + ((7 - 1 - now.getDay() + 7) % 7) + 1);
    nextSunday.setHours(8, 0, 0, 0);
    return nextSunday;
  }

  /**
   * Convert a UTC Date object to Eastern.
   */
  function convertToEST(date) {
    var estOffset = -5 + 1; // -4.0 daylight savings;
    var utc = date.getTime() + date.getTimezoneOffset() * 60000;
    return new Date(utc + 3600000 * estOffset);
  }

  initializeClock("clockdiv");

})();

countdown.css

.has-time {
  position: relative;
}

.is-time {
  display: flex;
  align-items: baseline;
  flex-wrap: wrap;
  font-size: 5rem;
  gap: 0;
  justify-content: center;
  _padding: 1.25rem;
  position: relative;
}

/**
* Project : simply-countdown
* File : simplyCountdown.theme.default
* Date : 27/06/2015
* Author : Vincent Loy <[email protected]>
*/
.simply-countdown.is-box > .simply-section {
  background: rgba(255, 255, 255, 0.3);
  border-radius: 2px;
  box-shadow: 0 0 3px rgba(0, 0, 0, 0.12), 0 0 2px rgba(0, 0, 0, 0.24);
}
.simply-countdown.is-rounded > .simply-section {
  background: rgba(255, 255, 255, 0.3);
  border-radius: 100%;
  box-shadow: 0 0 3px rgba(0, 0, 0, 0.12), 0 0 2px rgba(0, 0, 0, 0.24);
  height: min(7.75rem);
}
.simply-countdown.no-dividers > .simply-section:not(:last-child)::after {
  display: none;
}
.simply-countdown > .simply-section {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  margin: 0;
  padding: 1rem;
  position: relative;
  width: min(7.75rem);
}
.simply-countdown > .simply-section:not(:last-child)::after {
  content: "";
  display: none;
  background: currentColor;
  height: 100px;
  position: absolute;
  top: auto;
  left: 100%;
  transform: translate(100%, 0);
  width: 2px;
  z-index: 1;
}
@media (min-width: 768px) {
  .simply-countdown > .simply-section:not(:last-child)::after {
    display: block;
  }
}
.simply-countdown > .simply-section .simply-amount {
  font-weight: 700;
}
.simply-countdown > .simply-section .simply-word {
  font-size: 1rem;
  text-transform: uppercase;
}
.simply-countdown > .simply-section .simply-amount,
.simply-countdown > .simply-section .simply-word {
  display: block;
  text-align: center;
}

countdown.js

/**
 * @link: https://stackoverflow.com/questions/35587387/javascript-countdown-clock-every-saturday-at-11am#answer-35588057
 */

function getTimeRemaining(endtime) {
  var t = Date.parse(endtime) - Date.parse(new Date());
  var seconds = Math.floor((t / 1000) % 60);
  var minutes = Math.floor((t / 1000 / 60) % 60);
  var hours = Math.floor((t / (1000 * 60 * 60)) % 24);
  var days = Math.floor(t / (1000 * 60 * 60 * 24));
  return {
    total: t,
    days: days,
    hours: hours,
    minutes: minutes,
    seconds: seconds
  };
}

function initializeClock(id, endtime) {
  var clock = document.getElementById(id);
  var daysSpan = clock.querySelector(".days");
  var hoursSpan = clock.querySelector(".hours");
  var minutesSpan = clock.querySelector(".minutes");
  var secondsSpan = clock.querySelector(".seconds");

  function updateClock() {
    var t = getTimeRemaining(endtime);

    daysSpan.innerHTML = t.days;
    hoursSpan.innerHTML = ("0" + t.hours).slice(-2);
    minutesSpan.innerHTML = ("0" + t.minutes).slice(-2);
    secondsSpan.innerHTML = ("0" + t.seconds).slice(-2);

    if (t.total <= 0) {
      clearInterval(timeinterval);
    }
  }

  updateClock();
  var timeinterval = setInterval(updateClock, 1000);
}

function getNextSunday() {
  var now = new Date();
  var nextSunday = new Date();
  nextSunday.setDate(now.getDate() + ((7 - 1 - now.getDay() + 7) % 7) + 1);
  nextSunday.setHours(8, 0, 0, 0);
  return nextSunday;
}

function convertToEST(date) {
  var estOffset = -5 + 1; // -4.0 daylight savings;
  var utc = date.getTime() + date.getTimezoneOffset() * 60000;
  return new Date(utc + 3600000 * estOffset);
}

// var deadline = getNextSunday();
var deadline = new getNextSunday(
  Date.parse(new Date()) + 8 * 0 * 60 * 60 * 1000
);
initializeClock("clockdiv", convertToEST(deadline));