JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/moment.js/2.5.0/moment.js"></script>
<div class="datetime"></div>

JavaScript

var datetime = $('.datetime'),

//Update countdown
update = function(){
        var now = moment(),
            nextmonday = moment().day(1).startOf('day').add('day',7),
            diffSec = nextmonday.diff(now, 'seconds'),
            hours = Math.floor(diff / 3600),
            diff = diff - hours * 3600,
            minutes = Math.floor(diff / 60),
            seconds = diff - minutes * 60;

        datetime.html(diffSec);

    };

//If it's Monday do nothing
if (moment().day()!=1){
    update();
    setInterval(update, 1000);
} else {
    datetime.text('It\'s Monday!');
}