Timestamp converter.

What a Unix timestamp actually is

A single integer: the number of seconds that have elapsed since 00:00:00 UTC on January 1, 1970 (the "Unix epoch"). That's it — no timezone, no format, no ambiguity. 0 is the epoch itself. 1735689600 is the start of 2025. 1776883423 is somewhere near the moment this page was rendered.

Because the timestamp is an integer, it's trivial to store (one number), compare (larger = later), sort (integer ordering = chronological ordering), and transmit across systems that might disagree about timezones. That's why it's the default internal representation for dates in Unix, Linux, macOS, PostgreSQL, every JavaScript runtime, and countless APIs.

Seconds vs milliseconds (the gotcha)

Most Unix tools use seconds. JavaScript's Date.now() returns milliseconds. Some JSON APIs mix the two depending on who implemented them. The shortcut to tell them apart: a seconds timestamp for "now" is ~1,700,000,000 (10 digits). A milliseconds timestamp is ~1,700,000,000,000 (13 digits). If you're off by a factor of 1000, you probably mixed the two.

Converting in popular languages

// JavaScript
const seconds = Math.floor(Date.now() / 1000);
const date = new Date(seconds * 1000);          // to Date
const iso = date.toISOString();                 // "2026-04-22T..."

// Python
import time, datetime
seconds = int(time.time())
dt = datetime.datetime.fromtimestamp(seconds)   # local
dt_utc = datetime.datetime.utcfromtimestamp(seconds)

// Bash (Linux / GNU coreutils)
date +%s                           # current epoch
date -d @1735689600                # to readable
date -d "2026-04-22 12:00:00" +%s  # to epoch

// macOS / BSD bash
date +%s
date -r 1735689600                 # to readable

// PostgreSQL
SELECT EXTRACT(EPOCH FROM NOW())::int;
SELECT to_timestamp(1735689600);

// MySQL
SELECT UNIX_TIMESTAMP();
SELECT FROM_UNIXTIME(1735689600);

The Year 2038 problem

A 32-bit signed integer can hold values up to 2,147,483,647. That number of seconds after January 1, 1970 is 2038-01-19 03:14:07 UTC. After that moment, 32-bit Unix systems will wrap to negative and think it's 1901. The fix is 64-bit time_t, which most modern systems (macOS, Linux on 64-bit, Windows) already use. Legacy embedded devices and old databases are the remaining risk — 13 years from now, expect a wave of quiet failures in the least-maintained corners of the software world.

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