diff --git a/lib/posts-handler.js b/lib/posts-handler.js
index f03e3c4..2401ad0 100644
--- a/lib/posts-handler.js
+++ b/lib/posts-handler.js
@@ -1,92 +1,111 @@
-'use strict';
-const pug = require('pug');
-const { PrismaClient } = require('@prisma/client');
-const prisma = new PrismaClient();
-const util = require('./handler-util');
+'use strict'
+const pug = require('pug')
+const { PrismaClient } = require('@prisma/client')
+const prisma = new PrismaClient()
+const util = require('./handler-util')
+
+const dayjs = require('dayjs')
+const utc = require('dayjs/plugin/utc')
+const timezone = require('dayjs/plugin/timezone')
+const relativeTime = require('dayjs/plugin/relativeTime')
+require('dayjs/locale/ja')
+dayjs.locale('ja')
+dayjs.extend(utc)
+dayjs.extend(timezone)
+dayjs.extend(relativeTime)
+dayjs.tz.setDefault('Asia/Tokyo')
 
 async function handle(req, res) {
   switch (req.method) {
     case 'GET':
       res.writeHead(200, {
-        'Content-Type': 'text/html; charset=utf-8'
-      });
+        'Content-Type': 'text/html; charset=utf-8',
+      })
       const posts = await prisma.post.findMany({
         orderBy: {
-          id: 'asc'
-        }
-      });
+          id: 'asc',
+        },
+      })
       posts.forEach((post) => {
-        post.content = post.content.replace(/\n/g, '<br>');
-      });
-      res.end(pug.renderFile('./views/posts.pug', { posts, user: req.user }));
+        post.content = post.content.replace(/\n/g, '<br>')
+        post.relativeCreatedAt = dayjs(post.createdAt).tz().fromNow()
+        post.formattedCreatedAt = dayjs(post.createdAt)
+          .tz()
+          .format('YYYY年MM月DD日 HH時mm分ss秒')
+      })
+      res.end(pug.renderFile('./views/posts.pug', { posts, user: req.user }))
       console.info(
         `閲覧されました: user: ${req.user}, ` +
-        `remoteAddress: ${req.socket.remoteAddress}, ` +
-        `userAgent: ${req.headers['user-agent']} `
-      );
-      break;
+          `remoteAddress: ${req.socket.remoteAddress}, ` +
+          `userAgent: ${req.headers['user-agent']} `
+      )
+      break
     case 'POST':
-      let body = '';
-      req.on('data', (chunk) => {
-        body += chunk;
-      }).on('end', async () => {
-        const params = new URLSearchParams(body);
-        const content = params.get('content');
-        console.info(`送信されました: ${content}`);
-        await prisma.post.create({
-          data: {
-            content,
-            postedBy: req.user
-          }
-        });
-        handleRedirectPosts(req, res);
-      });
-      break;
+      let body = ''
+      req
+        .on('data', (chunk) => {
+          body += chunk
+        })
+        .on('end', async () => {
+          const params = new URLSearchParams(body)
+          const content = params.get('content')
+          console.info(`送信されました: ${content}`)
+          await prisma.post.create({
+            data: {
+              content,
+              postedBy: req.user,
+            },
+          })
+          handleRedirectPosts(req, res)
+        })
+      break
     default:
-      util.handleBadRequest(req, res);
-      break;
+      util.handleBadRequest(req, res)
+      break
   }
 }
 
 function handleRedirectPosts(req, res) {
   res.writeHead(303, {
-    'Location': '/posts'
-  });
-  res.end();
+    Location: '/posts',
+  })
+  res.end()
 }
 
 function handleDelete(req, res) {
   switch (req.method) {
     case 'POST':
-      let body = '';
-      req.on('data', (chunk) => {
-        body += chunk;
-      }).on('end', async () => {
-        const params = new URLSearchParams(body);
-        const id = parseInt(params.get('id'));
-        const post = await prisma.post.findUnique({
-          where: { id }
-        });
-        if (req.user === post.postedBy || req.user === 'admin') {
-          await prisma.post.delete({
-            where: { id }
-          });
-          console.info(
-            `削除されました: user: ${req.user}, ` +
-              `remoteAddress: ${req.socket.remoteAddress}, ` +
-              `userAgent: ${req.headers['user-agent']} `
-          );
-          handleRedirectPosts(req, res);
-        }
-      });
-      break;
+      let body = ''
+      req
+        .on('data', (chunk) => {
+          body += chunk
+        })
+        .on('end', async () => {
+          const params = new URLSearchParams(body)
+          const id = parseInt(params.get('id'))
+          const post = await prisma.post.findUnique({
+            where: { id },
+          })
+          if (req.user === post.postedBy || req.user === 'admin') {
+            await prisma.post.delete({
+              where: { id },
+            })
+            console.info(
+              `削除されました: user: ${req.user}, ` +
+                `remoteAddress: ${req.socket.remoteAddress}, ` +
+                `userAgent: ${req.headers['user-agent']} `
+            )
+            handleRedirectPosts(req, res)
+          }
+        })
+      break
     default:
-      util.handleBadRequest(req, res);
-      break;
+      util.handleBadRequest(req, res)
+      break
   }
 }
 
 module.exports = {
   handle,
-  handleDelete
-};
\ No newline at end of file
+  handleDelete,
+}
diff --git a/package.json b/package.json
index 9afc3ee..0ad6a7d 100644
--- a/package.json
+++ b/package.json
@@ -9,6 +9,7 @@
   "license": "MIT",
   "dependencies": {
     "@prisma/client": "4.11.0",
+    "dayjs": "1.11.7",
     "http-auth": "4.2.0",
     "prisma": "4.11.0",
     "pug": "3.0.2"
diff --git a/public/nn-chat.js b/public/nn-chat.js
index 08b7cee..3834113 100644
--- a/public/nn-chat.js
+++ b/public/nn-chat.js
@@ -1,31 +1,39 @@
-'use strict';
+'use strict'
 // 一番下を表示
-window.onload = function() {
-  window.scrollTo(0,document.body.scrollHeight);
+window.onload = function () {
+  window.scrollTo(0, document.body.scrollHeight)
 }
 
 // エンターキー と Ctrlキー(Macの場合はCommandキー)を押していたら送信
-const formElement = document.forms['message-form'];
-const textareaElement = formElement.elements['content'];
+const formElement = document.forms['message-form']
+const textareaElement = formElement.elements['content']
 textareaElement.addEventListener('keydown', (event) => {
   // 送信キーを押したら
   if (isPressedSubmitKey(event)) {
     // キーボード入力をキャンセルして送信
-    event.preventDefault();
-    formElement.submit();
+    event.preventDefault()
+    formElement.submit()
   }
-});
+})
 
 // 送信キーを押しているか判定
 function isPressedSubmitKey(event) {
   if (event.key !== 'Enter') {
-    return false;
+    return false
   }
   if (event.ctrlKey) {
-    return true;
+    return true
   }
   // MacのCommandキーはmetaKeyという名前
   if (event.metaKey) {
-    return true;
+    return true
   }
-}
\ No newline at end of file
+}
+
+// ツールチップの有効化
+const tooltipTriggerElements = document.querySelectorAll(
+  '[data-bs-toggle="tooltip"]'
+)
+tooltipTriggerElements.forEach((tooltipTriggerElement) => {
+  new bootstrap.Tooltip(tooltipTriggerElement)
+})
diff --git a/views/posts.pug b/views/posts.pug
index d9f2b14..386ac77 100644
--- a/views/posts.pug
+++ b/views/posts.pug
@@ -29,7 +29,8 @@ html(lang="ja")
             h5.card-title #{post.postedBy}
               if post.postedBy === 'admin'
                 i.bi-patch-check-fill.ms-1
-            small.card-text.text-muted.float-end #{post.createdAt}
+              - const tooltipTitle = `<small>${post.formattedCreatedAt}</small>`
+            small.card-text.text-muted.float-end(data-bs-toggle="tooltip" data-bs-placement="top" data-bs-html="true" title=tooltipTitle) #{post.relativeCreatedAt}
           p.card-text.lead!= post.content
         - const isDeletable = (user === post.postedBy || user === 'admin')
         if isDeletable