diff --git a/assets/js/__tests__/timeago.spec.ts b/assets/js/__tests__/timeago.spec.ts
new file mode 100644
index 00000000..e69e2702
--- /dev/null
+++ b/assets/js/__tests__/timeago.spec.ts
@@ -0,0 +1,114 @@
+import { timeAgo, setupTimestamps } from '../timeago';
+
+const epochRfc3339 = '1970-01-01T00:00:00.000Z';
+
+describe('Timeago functionality', () => {
+  // TODO: is this robust? do we need e.g. timekeeper to freeze the time?
+  function timeAgoWithSecondOffset(offset: number) {
+    const utc = new Date(new Date().getTime() + offset * 1000).toISOString();
+
+    const timeEl = document.createElement('time');
+    timeEl.setAttribute('datetime', utc);
+    timeEl.textContent = utc;
+
+    timeAgo([timeEl]);
+    return timeEl.textContent;
+  }
+
+  /* eslint-disable no-implicit-coercion */
+  it('should parse a time as less than a minute', () => {
+    expect(timeAgoWithSecondOffset(-15)).toEqual('less than a minute ago');
+    expect(timeAgoWithSecondOffset(+15)).toEqual('less than a minute from now');
+  });
+
+  it('should parse a time as about a minute', () => {
+    expect(timeAgoWithSecondOffset(-75)).toEqual('about a minute ago');
+    expect(timeAgoWithSecondOffset(+75)).toEqual('about a minute from now');
+  });
+
+  it('should parse a time as 30 minutes', () => {
+    expect(timeAgoWithSecondOffset(-(60 * 30))).toEqual('30 minutes ago');
+    expect(timeAgoWithSecondOffset(+(60 * 30))).toEqual('30 minutes from now');
+  });
+
+  it('should parse a time as about an hour', () => {
+    expect(timeAgoWithSecondOffset(-(60 * 60))).toEqual('about an hour ago');
+    expect(timeAgoWithSecondOffset(+(60 * 60))).toEqual('about an hour from now');
+  });
+
+  it('should parse a time as about 6 hours', () => {
+    expect(timeAgoWithSecondOffset(-(60 * 60 * 6))).toEqual('about 6 hours ago');
+    expect(timeAgoWithSecondOffset(+(60 * 60 * 6))).toEqual('about 6 hours from now');
+  });
+
+  it('should parse a time as a day', () => {
+    expect(timeAgoWithSecondOffset(-(60 * 60 * 36))).toEqual('a day ago');
+    expect(timeAgoWithSecondOffset(+(60 * 60 * 36))).toEqual('a day from now');
+  });
+
+  it('should parse a time as 25 days', () => {
+    expect(timeAgoWithSecondOffset(-(60 * 60 * 24 * 25))).toEqual('25 days ago');
+    expect(timeAgoWithSecondOffset(+(60 * 60 * 24 * 25))).toEqual('25 days from now');
+  });
+
+  it('should parse a time as about a month', () => {
+    expect(timeAgoWithSecondOffset(-(60 * 60 * 24 * 35))).toEqual('about a month ago');
+    expect(timeAgoWithSecondOffset(+(60 * 60 * 24 * 35))).toEqual('about a month from now');
+  });
+
+  it('should parse a time as 3 months', () => {
+    expect(timeAgoWithSecondOffset(-(60 * 60 * 24 * 30 * 3))).toEqual('3 months ago');
+    expect(timeAgoWithSecondOffset(+(60 * 60 * 24 * 30 * 3))).toEqual('3 months from now');
+  });
+
+  it('should parse a time as about a year', () => {
+    expect(timeAgoWithSecondOffset(-(60 * 60 * 24 * 30 * 13))).toEqual('about a year ago');
+    expect(timeAgoWithSecondOffset(+(60 * 60 * 24 * 30 * 13))).toEqual('about a year from now');
+  });
+
+  it('should parse a time as 5 years', () => {
+    expect(timeAgoWithSecondOffset(-(60 * 60 * 24 * 30 * 12 * 5))).toEqual('5 years ago');
+    expect(timeAgoWithSecondOffset(+(60 * 60 * 24 * 30 * 12 * 5))).toEqual('5 years from now');
+  });
+  /* eslint-enable no-implicit-coercion */
+
+  it('should ignore time elements without a datetime attribute', () => {
+    const timeEl = document.createElement('time');
+    const value = Math.random().toString();
+
+    timeEl.textContent = value;
+    timeAgo([timeEl]);
+
+    expect(timeEl.textContent).toEqual(value);
+  });
+
+  it('should not reset title attribute if it already exists', () => {
+    const timeEl = document.createElement('time');
+    const value = Math.random().toString();
+
+    timeEl.setAttribute('datetime', epochRfc3339);
+    timeEl.setAttribute('title', value);
+    timeAgo([timeEl]);
+
+    expect(timeEl.getAttribute('title')).toEqual(value);
+    expect(timeEl.textContent).not.toEqual(epochRfc3339);
+  });
+});
+
+describe('Automatic timestamps', () => {
+  it('should process all timestamps in the document', () => {
+    for (let i = 0; i < 5; i += 1) {
+      const timeEl = document.createElement('time');
+      timeEl.setAttribute('datetime', epochRfc3339);
+      timeEl.textContent = epochRfc3339;
+
+      document.documentElement.insertAdjacentElement('beforeend', timeEl);
+    }
+
+    setupTimestamps();
+
+    for (const timeEl of document.getElementsByTagName('time')) {
+      expect(timeEl.textContent).not.toEqual(epochRfc3339);
+    }
+  });
+});
diff --git a/assets/js/comment.js b/assets/js/comment.js
index f7b3797e..de245fa8 100644
--- a/assets/js/comment.js
+++ b/assets/js/comment.js
@@ -6,6 +6,7 @@ import { $ } from './utils/dom';
 import { showOwnedComments } from './communications/comment';
 import { filterNode } from './imagesclientside';
 import { fetchHtml } from './utils/requests';
+import { timeAgo } from './timeago';
 
 function handleError(response) {
 
@@ -91,7 +92,7 @@ function insertParentPost(data, clickedLink, fullComment) {
   fullComment.previousSibling.classList.add('fetched-comment');
 
   // Execute timeago on the new comment - it was not present when first run
-  window.booru.timeAgo(fullComment.previousSibling.getElementsByTagName('time'));
+  timeAgo(fullComment.previousSibling.getElementsByTagName('time'));
 
   // Add class active_reply_link to the clicked link
   clickedLink.classList.add('active_reply_link');
@@ -125,7 +126,7 @@ function displayComments(container, commentsHtml) {
   container.innerHTML = commentsHtml;
 
   // Execute timeago on comments
-  window.booru.timeAgo(document.getElementsByTagName('time'));
+  timeAgo(document.getElementsByTagName('time'));
 
   // Filter images in the comments
   filterNode(container);
diff --git a/assets/js/timeago.js b/assets/js/timeago.ts
similarity index 67%
rename from assets/js/timeago.js
rename to assets/js/timeago.ts
index f3694d6f..12f39bd9 100644
--- a/assets/js/timeago.js
+++ b/assets/js/timeago.ts
@@ -2,7 +2,9 @@
  * Frontend timestamps.
  */
 
-const strings = {
+import { assertNotNull } from './utils/assert';
+
+const strings: Record<string, string> = {
   seconds: 'less than a minute',
   minute: 'about a minute',
   minutes: '%d minutes',
@@ -16,16 +18,21 @@ const strings = {
   years: '%d years',
 };
 
-function distance(time) {
-  return new Date() - time;
+function distance(time: Date) {
+  return new Date().getTime() - time.getTime();
 }
 
-function substitute(key, amount) {
-  return strings[key].replace('%d', Math.round(amount));
+function substitute(key: string, amount: number) {
+  return strings[key].replace('%d', Math.round(amount).toString());
 }
 
-function setTimeAgo(el) {
-  const date = new Date(el.getAttribute('datetime'));
+function setTimeAgo(el: HTMLTimeElement) {
+  const datetime = el.getAttribute('datetime');
+  if (!datetime) {
+    return;
+  }
+
+  const date = new Date(datetime);
   const distMillis = distance(date);
 
   const seconds = Math.abs(distMillis) / 1000,
@@ -49,20 +56,20 @@ function setTimeAgo(el) {
                      substitute('years', years);
 
   if (!el.getAttribute('title')) {
-    el.setAttribute('title', el.textContent);
+    el.setAttribute('title', assertNotNull(el.textContent));
   }
   el.textContent = words + (distMillis < 0 ? ' from now' : ' ago');
 }
 
-function timeAgo(args) {
-  [].forEach.call(args, el => setTimeAgo(el));
+export function timeAgo(args: HTMLTimeElement[] | HTMLCollectionOf<HTMLTimeElement>) {
+  for (const el of args) {
+    setTimeAgo(el);
+  }
 }
 
-function setupTimestamps() {
+export function setupTimestamps() {
   timeAgo(document.getElementsByTagName('time'));
   window.setTimeout(setupTimestamps, 60000);
 }
 
-export { setupTimestamps };
-
 window.booru.timeAgo = timeAgo;
diff --git a/assets/test/jest-setup.ts b/assets/test/jest-setup.ts
index 82c545f8..29779672 100644
--- a/assets/test/jest-setup.ts
+++ b/assets/test/jest-setup.ts
@@ -2,6 +2,8 @@ import '@testing-library/jest-dom';
 import { matchNone } from '../js/query/boolean';
 
 window.booru = {
+  // eslint-disable-next-line @typescript-eslint/no-empty-function
+  timeAgo: () => {},
   csrfToken: 'mockCsrfToken',
   hiddenTag: '/mock-tagblocked.svg',
   hiddenTagList: [],
diff --git a/assets/types/booru-object.d.ts b/assets/types/booru-object.d.ts
index b4aead08..22d1aa08 100644
--- a/assets/types/booru-object.d.ts
+++ b/assets/types/booru-object.d.ts
@@ -13,6 +13,13 @@ interface Interaction {
 }
 
 interface BooruObject {
+  /**
+   * Automatic timestamp recalculation function for userscript use
+   */
+  timeAgo: (args: HTMLTimeElement[]) => void;
+  /**
+   * Anti-forgery token sent by the server
+   */
   csrfToken: string;
   /**
    * One of the specified values, based on user setting