JSFiddle - React, Tailwind, and code Playground

HTML

<div id="countdown" ></div>
<div id="countdown1" ></div>

JavaScript

var d = [5,4,3,2,1,0,7,6,];

var today = new Date();
var MOSCOW_UTC_OFFSET=180;
var ONE_MINUTE_IN_MILLISECONDS=60*1000;
var currentUTCTime=today.getTime()+today.getTimezoneOffset()*ONE_MINUTE_IN_MILLISECONDS;
var timesZoneModifier=currentUTCTime+MOSCOW_UTC_OFFSET*ONE_MINUTE_IN_MILLISECONDS;
var currentMoscowTime=new Date(timesZoneModifier);

console.log(currentMoscowTime);




var end = new Date(today.getFullYear(),today.getMonth(),today.getDate()+ d[today.getDay()], 15, 00, 00);
var _second = 1000;
var _minute = _second * 60;
var _hour = _minute * 60;
var _day = _hour * 24;
var timer;

    function showRemaining() {
        var now = new Date();
        var distance = end - now;
   if (distance < 0) {
           document.getElementById("countdown").innerHTML = "Следующий синхрон через неделю.";;
		   
            return;
        }
	
        var days = Math.floor(distance / _day);
        var hours = Math.floor((distance % _day) / _hour);
		if (hours < 10) hours = '0' + hours;
        var minutes = Math.floor((distance % _hour) / _minute);
		if (minutes < 10) minutes = '0' + minutes;
        var seconds = Math.floor((distance % _minute) / _second);
		if (seconds < 10) seconds = '0' + seconds;
	
        document.getElementById('countdown1').innerHTML = "До начала синхрона осталось. "+ days +" дн. "+ hours +":"+ minutes +":"+ seconds +"";
		
    }

    timer = setInterval(showRemaining, 1000);