< Last-Modified: Tue, 15 Nov 2026 12:45:26 GMT

TL;DR: Indicates when a resource was last changed using a timestamp. Enables cache validation with If-Modified-Since for efficient bandwidth usage.

What is Last-Modified?

Last-Modified tells browsers when a resource was last changed. It’s like a timestamp on a file that says “this was updated on January 15th at 3:42 PM.” Browsers use this information to check if their cached version is still current without downloading the entire resource again.

This header enables efficient cache validation and reduces unnecessary data transfers.

How It Works

1. Server sends resource with timestamp:

HTTP/1.1 200 OK
Last-Modified: Wed, 15 Jan 2025 15:42:30 GMT
Content-Type: text/html

<html>...</html>

2. Browser caches the response and timestamp.

3. Later, browser asks “changed since then?”:

GET /page.html HTTP/1.1
If-Modified-Since: Wed, 15 Jan 2025 15:42:30 GMT

4. If unchanged, server responds:

HTTP/1.1 304 Not Modified
Last-Modified: Wed, 15 Jan 2025 15:42:30 GMT

5. Browser reuses cached version - saves bandwidth and time!

Date Format

Last-Modified uses HTTP date format (RFC 9110 section 5.6.7):

Last-Modified: Wed, 15 Jan 2025 15:42:30 GMT

Format: Day, DD Mon YYYY HH:MM:SS GMT

• Always in GMT/UTC timezone
• Three-letter day and month names
• 24-hour time format

Generating Last-Modified

File System

For static files, use file modification time:

const fs = require('fs')
const stats = fs.statSync('index.html')
const lastModified = stats.mtime.toUTCString()
// "Wed, 15 Jan 2025 15:42:30 GMT"

Database Records

Use record update timestamp:

SELECT *, updated_at FROM articles WHERE id = 1;

const lastModified = new Date(article.updated_at).toUTCString()

Dynamic Content

For generated content, use the most recent data timestamp:

// User dashboard - use latest activity
const lastActivity = Math.max(
  user.updated_at,
  user.last_login,
  ...user.notifications.map((n) => n.created_at)
)
const lastModified = new Date(lastActivity).toUTCString()

Application Deployment

For app resources, use build/deploy time:

const buildTime = process.env.BUILD_TIMESTAMP || Date.now()
const lastModified = new Date(buildTime).toUTCString()

Conditional Requests

If-Modified-Since (GET requests)

Check if resource changed since cached version:

GET /api/posts HTTP/1.1
If-Modified-Since: Wed, 15 Jan 2025 15:42:30 GMT

Server responses:

• 304 Not Modified - No changes, use cached version
• 200 OK - Resource changed, here’s new content

If-Unmodified-Since (PUT/DELETE requests)

Prevent lost updates by ensuring resource hasn’t changed:

PUT /api/posts/123 HTTP/1.1
If-Unmodified-Since: Wed, 15 Jan 2025 15:42:30 GMT
Content-Type: application/json

{"title": "Updated Post"}

Server responses:

• 200 OK - Resource unchanged, update successful
• 412 Precondition Failed - Resource was modified by someone else

Cache Validation Flow

Initial Request:

GET /style.css HTTP/1.1

HTTP/1.1 200 OK
Last-Modified: Wed, 15 Jan 2025 10:30:00 GMT
Cache-Control: max-age=3600
Content-Type: text/css

body { color: blue; }

After Cache Expires:

GET /style.css HTTP/1.1
If-Modified-Since: Wed, 15 Jan 2025 10:30:00 GMT

HTTP/1.1 304 Not Modified
Last-Modified: Wed, 15 Jan 2025 10:30:00 GMT
Cache-Control: max-age=3600

Browser reuses cached CSS for another hour.

Performance Benefits

Bandwidth Savings

File unchanged scenario:
- Without Last-Modified: 50KB transfer
- With Last-Modified: ~200 bytes (304 response)
- Savings: 99.6%

Faster Page Loads

• No content download for unchanged resources
• Instant cache revalidation
• Reduced server load

Real-World Examples

Blog Posts

GET /posts/my-article HTTP/1.1

HTTP/1.1 200 OK
Last-Modified: Mon, 10 Jan 2025 14:20:00 GMT
Content-Type: text/html

<article>...</article>

API Data

GET /api/users/profile HTTP/1.1

HTTP/1.1 200 OK
Last-Modified: Tue, 11 Jan 2025 09:15:30 GMT
Content-Type: application/json

{"name": "John", "email": "john@example.com"}

Static Assets

GET /images/logo.png HTTP/1.1

HTTP/1.1 200 OK
Last-Modified: Fri, 01 Jan 2025 12:00:00 GMT
Content-Type: image/png

[PNG image data]

Last-Modified vs ETag

Feature	Last-Modified	ETag
Precision	1-second resolution	Exact change detection
Generation	Use timestamp	Hash content or version
Reliability	Can have clock issues	Always accurate
Performance	Faster to generate	More CPU to generate
Use Case	File-based content	Dynamic/API content

Using Both Together

HTTP/1.1 200 OK
Last-Modified: Wed, 15 Jan 2025 15:42:30 GMT
ETag: "abc123"
Content-Type: application/json

{"data": "..."}

Browser will use ETag if available, fall back to Last-Modified.

Getting Last-Modified right

1. Always include for cacheable content:

Last-Modified: Wed, 15 Jan 2025 15:42:30 GMT
Cache-Control: max-age=3600

2. Use precise timestamps:

// ✅ Use actual modification time
const lastModified = file.mtime.toUTCString()

// ❌ Don't use current time
const lastModified = new Date().toUTCString()

3. Handle conditional requests:

app.get('/api/data', (req, res) => {
  const lastModified = data.updated_at.toUTCString()
  const ifModifiedSince = req.headers['if-modified-since']

  if (ifModifiedSince === lastModified) {
    return res.status(304).end()
  }

  res.setHeader('Last-Modified', lastModified)
  res.json(data)
})

4. Use with optimistic locking:

app.put('/api/posts/:id', (req, res) => {
  const post = getPost(req.params.id)
  const lastModified = post.updated_at.toUTCString()
  const ifUnmodifiedSince = req.headers['if-unmodified-since']

  if (ifUnmodifiedSince !== lastModified) {
    return res.status(412).json({ error: 'Post was modified' })
  }

  // Safe to update
  updatePost(req.params.id, req.body)
})

Common Issues

Problem: Clock synchronization across servers Solution: Use database timestamps or centralized time source

Problem: 1-second precision not enough Solution: Use ETag for sub-second changes

Problem: Timezone confusion Solution: Always use GMT/UTC in HTTP headers

Related Headers

• If-Modified-Since - Conditional request header
• If-Unmodified-Since - Conditional request for updates
• ETag - More precise cache validation
• Cache-Control - Caching directives