JSFiddle - React, Tailwind, and code Playground

JavaScript

function treatAsUTC(date) {
    var result = new Date(date);
    result.setMinutes(result.getMinutes() - result.getTimezoneOffset());
    return result;
}

function daysBetween(startDate, endDate) {
    var millisecondsPerDay = 24 * 60 * 60 * 1000;
    return Math.floor((treatAsUTC(endDate) - treatAsUTC(startDate)) / millisecondsPerDay);
}

var demarcationdate = new Date("2016-12-19"), 
    today = new Date(),
    days = daysBetween(demarcationdate,today),
    daystill = 14 - days%14,
    rec = days%14==0;

alert("Days diff = "+days+". Recurs today= "+rec+". Next in "+daystill+" days.");