CynicmansionTime

var authorPostsTime = Array
    .from(document.querySelectorAll(".b-comment__user"))
    .filter(_=>_.querySelector(".b-comment__user_author_story"))
    .map(_=>_.querySelector(".b-comment__time"))
    .map(_=>_.getAttribute("datetime"))
    .map(_=>new Date(+_*1000));
var commentTime = new Date(Math.min(...authorPostsTime));

void new Promise(resolve => {
    var xhr = new XMLHttpRequest();
    xhr.responseType = 'document';
    xhr.open("GET", location.href);
    xhr.onreadystatechange = () => xhr.readyState == 4 && resolve(xhr);
    xhr.send();
})
    .then(_=>_.responseXML)
    .then(_=>_.querySelector("div.story__date[title]"))
    .then(_=>_.getAttribute("title"))
    .then(_=>new Date(+_*1000))
    .then(postTime => console.table({
        "Пост":  {"value":postTime.toLocaleString()},
        "Комментарий": {"value":commentTime.toLocaleString()},
        "Ранзица, секунд": {"value":(commentTime-postTime)/1000}
    }));