JSFiddle - React, Tailwind, and code Playground

HTML

<div id="counter">Содержимое контейнера должно быть таймером</div>
<ul class="number">
	<li><span>лет</span>11</li><li><span>месяцев</span>7</li><li><span>день</span>21</li><li><span>секунд</span>56</li>
</ul>

JavaScript

function CountUp(initDate, id) {
    this.beginDate = new Date(initDate);
    this.countainer = document.getElementById(id);
    this.numOfDays = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
    this.borrowed = 0;
    this.years = 0;
    this.months = 0;
    this.days = 0;
    this.hours = 0;
    this.minutes = 0;
    this.seconds = 0;
    this.localize_rus = {
        'years': ['год', 'года', 'лет'],
        'months': ['месяц', 'месяца', 'месяцев'],
        'days': ['день', 'дня', 'дней'],
        'hours': ['час', 'часа', 'часов'],
        'minutes': ['минута', 'минуты', 'минут'],
        'seconds': ['секунда', 'секунды', 'секунд']
    };
    this.updateNumOfDays();
    this.updateCounter();

}

CountUp.prototype.updateNumOfDays = function() {
    var dateNow = new Date();
    var currYear = dateNow.getFullYear();
    if ((currYear % 4 == 0 && currYear % 100 != 0) || currYear % 400 == 0) {
        this.numOfDays[1] = 29;
    }
    var self = this;
    setTimeout(function() {
        self.updateNumOfDays();
    }, (new Date((currYear + 1), 1, 2) - dateNow));
};

CountUp.prototype.datePartDiff = function(then, now, MAX) {
    var diff = now - then - this.borrowed;
    this.borrowed = 0;
    if (diff > -1) return diff;
    this.borrowed = 1;
    return (MAX + diff);
};

CountUp.prototype.calculate = function() {
    var currDate = new Date();
    var prevDate = this.beginDate;
    this.seconds = this.datePartDiff(prevDate.getSeconds(), currDate.getSeconds(), 60);
    this.minutes = this.datePartDiff(prevDate.getMinutes(), currDate.getMinutes(), 60);
    this.hours = this.datePartDiff(prevDate.getHours(), currDate.getHours(), 24);
    this.days = this.datePartDiff(prevDate.getDate(), currDate.getDate(), this.numOfDays[currDate.getMonth()]);
    this.months = this.datePartDiff(prevDate.getMonth(), currDate.getMonth(), 12);
    this.years = this.datePartDiff(prevDate.getFullYear(), currDate.getFullYear(), 0);
};

CountUp.prototype.addLeadingZero = function(value)...