JSFiddle - React, Tailwind, and code Playground

HTML

Timer: <span id="countup1"><span class="value_timer">00</span></span>

CSS

span.digit {
	background: black;
	color: white;
	padding: 1px 3px;
	margin: 1px;
	
	font-family: var(--mainFontFamily);
	font-size: var(--mainFontSize);
 }

JavaScript

// TIMER Styling START
						function styleChars(targetEl) {
							const val = targetEl.textContent;
							const chars = val.split('');
							targetEl.innerHTML = chars.map(c => `<span class="digit">${c}</span>`).join('');
						}
						
						const target = document.querySelector('.value_timer');
						console.log(target);
						styleChars(target);
						// TIMER Styling END
						
						// TIMER START
						var timerDate = "January 01, 2020 14:00:00";
						
							function setTimer() {
								// Month Day, Year Hour:Minute:Second, id-of-element-container.
								countUpFromTime(timerDate, 'countup1');
							};
							
							window.onload = setTimer();
							
							function countUpFromTime(countFrom, id) {
								let countDate = new Date(countFrom);
								let now = new Date();
								let timeDifference = (now.getTime() - countDate.getTime());

								let value_timer = Math.floor(timeDifference / 1000 / 60 / 60);

								let idEl = document.getElementById(id);
								idEl.innerHTML = value_timer;
								styleChars(idEl); // Pass element to styling function.

								clearTimeout(countUpFromTime.interval);
								countUpFromTime.interval = setTimeout(function () {
									countUpFromTime(countFrom, id);
								}, 1000);
							}
					
						// TIMER END