What Is Unix Time? Complete Beginner Guide

What is Unix Time?

Unlike human-readable date formats (like December 5th, 2026) which have to account for months, years, leap years, and time zones, computers prefer simpler math.

Unix time represents time as a single, continuously increasing integer. This integer represents the exact number of seconds that have passed since a specific starting point. Because it's a simple integer, computers can compare, sort, and add/subtract time using extremely fast, low-level CPU instructions.

The Unix Epoch

In order to count elapsed seconds, you need a starting point. In the Unix operating system (created at Bell Labs in the early 1970s), engineers decided on an arbitrary starting date: January 1st, 1970 at 00:00:00 UTC.

This exact moment is known as the Unix Epoch.

• If a Unix timestamp is 0, the date is exactly Jan 1, 1970.
• If a Unix timestamp is 86400, exactly one day has passed: Jan 2, 1970.
• If a Unix timestamp is negative (e.g., -86400), it represents Dec 31, 1969.

Why Do We Use It?

Storing dates as strings (like 2026-05-21T14:30:00Z) takes up memory (at least 20 bytes for the string). In the 1970s and 1980s, memory was incredibly expensive. Storing time as a 32-bit integer took only 4 bytes.

Today, we still use it because of its raw performance and universality. If you want to know if an authentication token is expired, you simply check if currentTime > tokenExpiryTime. It's a single integer comparison, which takes a fraction of a nanosecond.

The Year 2038 Problem (Y2K38)

Because memory was so expensive, Unix time was originally designed to fit into a signed 32-bit integer.

The maximum value a signed 32-bit integer can hold is 2,147,483,647. If you add 2.14 billion seconds to January 1, 1970, you land on January 19th, 2038 at 03:14:07 UTC.

To fix this, modern systems have migrated to 64-bit integers for time storage. A 64-bit integer can store timestamps up to 292 billion years into the future.

Code Examples

// Get CURRENT Unix Time (in SECONDS)
const unixSeconds = Math.floor(Date.now() / 1000);

// Convert Unix Seconds back to a JS Date Object
// Note: JS requires MILLISECONDS, so you must multiply by 1000
const dateObject = new Date(unixSeconds * 1000);

import time
from datetime import datetime

# Get CURRENT Unix Time (in SECONDS)
unix_seconds = int(time.time())

# Convert Unix Seconds to DateTime object
date_object = datetime.fromtimestamp(unix_seconds)

Frequently Asked Questions