Tampermonkey extension that parses almost any text string in the specific format: TZ(+1) to UTC, so you can see what is the timezone of your friend living in mars,

discord-timezones.js

// ==UserScript==
// @name         Annotate Discord user timezones
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  try to take over the world!
// @author       You
// @match        https://discord.com/*
// @icon         https://icons.duckduckgo.com/ip3/discord.com.ico
// @grant        none
// ==/UserScript==

/**
 * function to calculate local time
 * in a different city
 * given the city's UTC offset
 */
function calcTime({ offsetHours, offsetMinutes, ahead = false }) {
  offsetHours = isNaN(offsetHours) ? 0 : offsetHours
  offsetMinutes = isNaN(offsetMinutes) ? 0 : offsetMinutes

  const dir = ahead ? 1 : -1
  offsetHours = Math.abs(offsetHours) * dir
  offsetMinutes = Math.abs(offsetMinutes) * dir

  // create Date object for current location
  var d = new Date();

  // get UTC time in msec
  var utc = d.getTime();

  // create new Date object for different city
  // using supplied offset
  var nd = new Date(utc + (3600000 * offsetHours + 60000 * offsetMinutes));

  // return time as a string
  return [nd.getUTCHours(), nd.getUTCMinutes()];
}

(function () {
  'use strict';

  // because discord developers thought they could hide localStorage from the browser, see:
  // https://stackoverflow.com/questions/52509440/discord-window-localstorage-is-undefined-how-to-get-access-to-the-localstorage
  function getLocalStoragePropertyDescriptor() {
    const iframe = document.createElement('iframe');
    document.head.append(iframe);
    const pd = Object.getOwnPropertyDescriptor(iframe.contentWindow, 'localStorage');
    iframe.remove();
    return pd;
  }

  // We have several options for how to use the property descriptor
  // once we have it. The simplest is to just redefine it:

  Object.defineProperty(window, 'localStorage', getLocalStoragePropertyDescriptor());

  window.addEventListener("load", function () {
    // create a new regex instance to avoid weird stuff happening.
    // i am not sure what is exactly but i remember suffering with it in the past.
    // and if you suck at regexes like me, use https://regexr.com/.
    const baseRegex = () => /.*TZ\((\+|-)?([0-9][0-9]?)(:([1-5][0-9]))?\)/g
    const tzRegex = () => /TZ\((\+|-)?([0-9][0-9]?)(:([1-5][0-9]))?\)/g
    const hourRegex = () => /\d\d:\d\d/g

    const selectors = 'tz_____selectors____discord____ext__tamp';
    const STOP_TRANSVERSE = 1 << 1

    function set(k, v) {
      localStorage.setItem(k, JSON.stringify(v))
    }

    function remove(k) {
      localStorage.removeItem(k)
    }

    function get(k) {
      return JSON.parse(localStorage.getItem(k) || "[]")
    }

    function looper() {
      console.log("Updating your friend timezones...")
      for (const selector of get(selectors)) {
        try {
          applyTzConversion("." + selector.split(" ").join("."))
        } catch(e) {
          console.log("Error: " + e)
          set(selectors, get(selectors).filter(e => e !== selector))
        }
      }

      setTimeout(looper, 1 * 1000)
    }

    function applyTzConversion(selector) {
      const elements = [...document.querySelectorAll(selector)]
      for (const element of elements) {
        transverse(element, function (node) {
          if (hasMatch(node.textContent) && node.children.length === 0) {
            const [fullmatch, tz, h, colon, m] = [...node.textContent.matchAll(baseRegex())][0]

            const ahead = !tz || tz === "+"
            const offsetHours = Number(h)
            const offsetMinutes = Number(m)

            const [hours, minutes] = calcTime({ ahead, offsetHours, offsetMinutes })

            if(hourRegex().test(node.textContent)) {
              node.textContent = node.textContent.replace(hourRegex(), `${hours.toString().padStart(2, 0)}:${minutes.toString().padStart(2, 0)}`)
            } else {
              node.textContent = node.textContent.replace(tzRegex(), `${hours.toString().padStart(2, 0)}:${minutes.toString().padStart(2, 0)} ${[...node.textContent.match(tzRegex())][0]}`)
            }

            return STOP_TRANSVERSE
          }
        })
      }
    }

    function transverse(node, callback) {
      if (callback(node) === STOP_TRANSVERSE) return

      for (const child of node.children) {
        transverse(child, callback)
      }
    }

    function hasMatch(text) {
      return baseRegex().test(text)
    }

    window.addEventListener("click", function (e) {
      if (hasMatch(e.target.innerText)) {
        set(selectors, [...new Set([...get(selectors), [...e.target.classList].join(' ')])])
      }
    })

    setTimeout(looper, 1 * 1000);
  })
})();