JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.18.1/moment.min.js"></script>

JavaScript

var last_renewed_date = moment([2017, 07, 29]);//aug 29

var next_renewal_date = last_renewed_date.add(1, 'month');//sep 29

today = moment(); //today is sep 9
var diff_in_days = next_renewal_date.diff(today, 'days');
console.clear();
console.log("last_renewed_date",last_renewed_date)
console.log("next_renewal_date",next_renewal_date)
console.log('diff_in_days', diff_in_days); //20 days left