Unix Timestamps Explained: Seconds, Milliseconds, and the Y2K38 Problem

Unix timestamps are the lingua franca of time in software. Every language can generate and parse them. But seconds vs milliseconds trips up almost every developer at least once — and the Y2K38 problem is coming.

What a Unix timestamp is

A Unix timestamp is the number of seconds (or milliseconds, depending on the system) that have elapsed since January 1, 1970, 00:00:00 UTC — the "Unix epoch." It's timezone-independent, which is why it's the standard for storing and transmitting time in systems.

// Current timestamp in different languages
JavaScript: Date.now()              // milliseconds → 1720000000000
Python:     time.time()             // seconds (float) → 1720000000.12
Go:         time.Now().Unix()       // seconds → 1720000000
Rust:       SystemTime::now()       // Duration since epoch
SQL:        EXTRACT(EPOCH FROM NOW()) // PostgreSQL, seconds

The seconds vs milliseconds problem

This is the most common source of timestamp bugs. JavaScript Date.now() returns milliseconds. Almost every other language and API returns seconds. Pass a millisecond timestamp to code expecting seconds and you get a date in the year 56,000.

// Detect: if > 9,999,999,999 it's probably milliseconds
const isMs = (ts: number) => ts > 9_999_999_999

// Convert JS Date.now() to seconds:
const seconds = Math.floor(Date.now() / 1000)

// Convert seconds to JS Date:
const date = new Date(seconds * 1000)

The Y2K38 problem

Many older systems store Unix timestamps as a signed 32-bit integer. The maximum value is 2,147,483,647 — which represents January 19, 2038 at 03:14:07 UTC. After that moment, a 32-bit signed integer wraps to a large negative number: January 13, 1901.

Systems at risk: embedded systems, 32-bit Linux systems, older databases, PHP time() on 32-bit platforms, MySQL TIMESTAMP column type (max 2038-01-19).

Fix: Use 64-bit integers for timestamps. In MySQL, use DATETIME instead of TIMESTAMP. In PostgreSQL, TIMESTAMP is already 64-bit and safe until year 294276.

Notable timestamps

Conversion across languages

// JavaScript — convert timestamp to human-readable
const ts = 1720000000
new Date(ts * 1000).toISOString()  // "2024-07-03T12:26:40.000Z"
new Date(ts * 1000).toLocaleString("en-US", { timeZone: "America/New_York" })

// Python
import datetime
datetime.datetime.fromtimestamp(1720000000, tz=datetime.timezone.utc)

// Go
time.Unix(1720000000, 0).UTC().Format(time.RFC3339)

// SQL (PostgreSQL)
SELECT to_timestamp(1720000000) AT TIME ZONE 'UTC';

Convert any Unix timestamp instantly — including timezone support and code snippets for all major languages — with the Unix Timestamp Converter.