Unix Timestamp Converter

What Unix time is

Unix time is the number of seconds elapsed since 1970-01-01T00:00:00Z — the "epoch." It is timezone-free, always monotonic (mostly), and takes a single integer to represent any moment for the next few thousand years.

Some systems use milliseconds instead of seconds. JavaScript's Date.now() returns milliseconds; most Unix system calls return seconds. Getting the unit wrong is the single most common bug.

Seconds vs. milliseconds — how to tell

Count the digits at 2025-06:

• Seconds: 10 digits (1_749_x_xxx_xxx).
• Milliseconds: 13 digits.

Anything between 10 and 12 digits is almost certainly seconds. Anything 13+ is almost certainly milliseconds.

Round-trip formulas

// JavaScript, ms → Date
const d = new Date(1749_000_000_000)

// JavaScript, seconds → Date
const d2 = new Date(1_749_000_000 * 1000)

// Date → Unix seconds
Math.floor(Date.now() / 1000)

// Date → Unix ms
Date.now()

ISO 8601 — the safest wire format

When you have to send a date across systems, use ISO 8601 in UTC ("Zulu") form:

2025-06-14T12:34:56Z

It is human-readable, sortable as a string, timezone-explicit, and every language's date parser understands it.

The 2038 problem

32-bit signed integers overflow on 19 Jan 2038, 03:14:07 UTC. Systems written before ~2010 that stored time as int32 will wrap around to 1901. Modern platforms use 64-bit integers — the sun will burn out first.

Check any legacy database columns that store Unix time as a 32-bit integer.

Common mistakes

1. Mixing seconds and milliseconds. Always label your columns / variables.
2. Assuming local time. Unix time is UTC. Always convert at the edges of your system.
3. Using new Date('2025-06-01'). That parses as UTC in some browsers, local in others. Use new Date('2025-06-01T00:00:00Z').
4. Storing dates as strings. Store timestamps as integers, format them at render time.
5. Ignoring leap seconds. POSIX Unix time pretends leap seconds don't exist. Rarely matters, worth knowing.

FAQ

Why 1970? Unix was designed in 1969-1970. The epoch was set to the first second of the year Unix shipped.

Does Unix time include leap seconds? No. POSIX defines Unix time as days_since_epoch * 86400 + seconds_in_day — leap seconds are silently skipped.

What is the largest Unix time? 2^63 - 1 on modern 64-bit systems — about 292 billion years from now.