Unix Timestamps Explained: Epoch Time, Seconds vs Milliseconds

What is a Unix Timestamp?

A Unix timestamp (also called epoch time or POSIX time) is the number of seconds that have elapsed since January 1, 1970, at 00:00:00 UTC. This reference point is called the Unix epoch.

Unix timestamps are timezone-independent — they represent an absolute point in time, not a local time. This makes them ideal for storing and comparing times across different time zones.

Seconds vs Milliseconds

Converting Timestamps

// JavaScript
const now = Date.now()              // milliseconds
const seconds = Math.floor(Date.now() / 1000)  // seconds
const date = new Date(1704067200000)  // from ms timestamp

# Python
import time, datetime
now = int(time.time())              # seconds
dt = datetime.datetime.fromtimestamp(1704067200)

// PHP
$now = time();                       // seconds
$date = date('Y-m-d', 1704067200);

The Year 2038 Problem

32-bit signed integers can store values up to 2,147,483,647. Unix timestamps will overflow this limit on January 19, 2038 at 03:14:07 UTC. Modern 64-bit systems are not affected — a 64-bit timestamp won't overflow for approximately 292 billion years.