Date.toDiffString.js

function declOfNum(number, titles) {
    number = Math.abs(number);
    return titles[(number % 100 > 4 && number % 100 < 20) ? 2 : [2, 0, 1, 1, 1, 2][(number % 10 < 5) ? number % 10 : 5]];
}

Date.prototype.DiffString = [
    ["секунду", "* секунды", "* секунд"],
    ["минуту", "* минуты", "* минут"],
    ["час", "* часа", "* часов"],
    ["день", "* дня", "* дней"],
    ["год", "* года", "* лет"],
    ["через *", "* назад", "прямо сейчас"]
];

Date.prototype.toDiffString = function (offset, lang) {
    lang = lang || Date.prototype.DiffString;
    offset = offset || [60, 3600, 86400, 31557600];
    var now = Date.now();
    var _this = this.getTime();
    var diff = Math.floor(Math.abs(now - _this) / 1000);

    var response = [];
    for (var i = 0; i < offset.length; i++) {
        response.push(diff % offset[i]);
        if(i > 0) response[i] = Math.floor(response[i] / offset[i - 1]);
        diff -= response[i];
    }
    response = response
        .map((v, i) => declOfNum(v, lang[i]).replace("*", v))
        .filter(v => v[0] !== "0")
        .reverse();
    if(!response.length) return lang[5][2];
    return lang[5][(now > _this) * 1].replace("*", response.join(" "));
};