JSFiddle - React, Tailwind, and code Playground

JavaScript

var start, finish, midpoint, percent, elapsed;

start = new Date(2017, 8, 22, 9);
finish = new Date(2017, 9, 30, 22);
midpoint = new Date(2017, 8, 29, 12);

elapsed = midpoint - start;
percent = (elapsed / (finish - start)) * 100;

console.log('elapsed', elapsed, ' ms', percent, ' % complete');