Решил сам:
function daysOnMarket() {
var li = document.getElementsByClassName("date-listed")[0];
var newLi = document.getElementsByClassName("date-listed")[0];
var oldDate = document.querySelector(".onmarket-date .field-value").innerText.replace(/(\d{2})\/(\d{2})\/(\d{4})/g, '$3-$1-$2');
var days = Math.floor((new Date() - new Date(oldDate))/(24 * 60 * 60 * 1000));
newLi.insertAdjacentHTML('afterend', '<li class="field days-on-market"><span class="field-name">Days on market: </span><span class="field-value">' + days + ' day(s)' + '</span></li>');
li.remove();
}
daysOnMarket();