fix-outlook-date.user.js

// ==UserScript==
// @name           fix-outlook-date
// @namespace      http://fix-outlook-date.whatev
// @version        0.1
// @author         lookout
// @description    fix-outlook-date
// @license        GPL version 3 or any later version; http://www.gnu.org/copyleft/gpl.html
// @require        https://cdnjs.cloudflare.com/ajax/libs/jquery/2.2.1/jquery.js
// @include        https://outlook*
// @include        https://webmail*
// @grant          none
// ==/UserScript==

if ($('body').filter('.ms-fwt-r[aria-label="Outlook"]').length) {
    $('._lvv_Q').has('span').each(function () {
        var text = $(this).text().trim(),
            msgDate = new Date(),
            dayNum = null;
        if (!text.includes('.')) {
            if (text.includes('Пн')) {
                dayNum = 1;
            } else if (text.includes('Вт')) {
                dayNum = 2;
            } else if (text.includes('Ср')) {
                dayNum = 3;
            } else if (text.includes('Чт')) {
                dayNum = 4;
            } else if (text.includes('Пт')) {
                dayNum = 5;
            } else if (text.includes('Сб')) {
                dayNum = 6;
            } else if (text.includes('Вс')) {
                dayNum = 7;
            } else if (text.includes(':')) {
                dayNum = msgDate.getDay();
            }
            if (Number.isInteger(dayNum)) {
                var dayShift = dayNum - msgDate.getDay();
                msgDate = new Date(msgDate.setDate(msgDate.getDate() + dayShift));
                $(this).prepend($(
                    '<span>' +
                    msgDate.getDate() + '.' + ('0' + (msgDate.getMonth() + 1)).slice(-2) + ' ' +
                    '</span>'
                ))
            }
        }
    })
}