What is Unix Time? A Complete Guide

Unix time (also known as epoch time, POSIX time, or Unix timestamp) is a system for tracking time as a running total of seconds. It counts the number of seconds that have elapsed since January 1, 1970, at 00:00:00 UTC, a moment known as the "Unix epoch."

How Does Unix Time Work?

Unix time is elegantly simple: it's just a number that increases by one every second. At the Unix epoch (January 1, 1970, 00:00:00 UTC), the timestamp was 0. One day later, it was 86400 (60 seconds × 60 minutes × 24 hours).

Key characteristics of Unix time:

• Always UTC: Unix timestamps are timezone-agnostic, always representing UTC time
• Linear: Time only moves forward (ignoring leap second adjustments)
• Compact: A single number represents a complete moment in time
• Universal: The same timestamp means the same instant everywhere in the world

Seconds vs. Milliseconds

The original Unix timestamp uses seconds, but many modern systems usemilliseconds for greater precision. Here's how to tell them apart:

JavaScript's Date.now() returns milliseconds, while Python'stime.time() returns seconds as a float.

Why Do Developers Use Unix Time?

Common Use Cases

• API timestamps: REST APIs often return dates as Unix timestamps
• Log files: System logs use epoch time for precise event ordering
• Caching: Cache expiration times are set using Unix timestamps
• JWT tokens: JSON Web Tokens use Unix time for iat and exp claims
• Database records: created_at and updated_at fields
• File systems: File modification times are stored as Unix timestamps

The Year 2038 Problem

Many older systems store Unix time as a 32-bit signed integer, which can represent dates up to January 19, 2038, 03:14:07 UTC. After this moment, the timestamp overflows and wraps around to a negative number, potentially causing system failures.

Modern systems use 64-bit integers, which can represent dates billions of years into the future, effectively solving this problem. However, legacy systems and embedded devices may still be vulnerable.

Getting Unix Time in Code

JavaScript

// Current time in milliseconds
Date.now()

// Current time in seconds
Math.floor(Date.now() / 1000)

// Specific date to Unix
new Date('2024-01-01').getTime() / 1000

Python

import time
from datetime import datetime

# Current time in seconds (float)
time.time()

# Current time in seconds (integer)
int(time.time())

# Specific date to Unix
datetime(2024, 1, 1).timestamp()

PHP

// Current time
time()

// Specific date to Unix
strtotime('2024-01-01')