How does it tell seconds from milliseconds?

Pure numbers with absolute value over 100 billion are treated as milliseconds; otherwise seconds. Dates before 1973 are the rare ambiguity zone.

Is the ISO output always UTC?

Yes; toISOString() is defined as UTC with Z suffix. Convert to your time zone at presentation time, never before storage.

Can I convert negative timestamps?

Yes; pre-1970 dates work as negative seconds/milliseconds throughout.

Why does my relative time say “55 years ago” for a small number?

You likely pasted milliseconds into what was interpreted as seconds (or vice versa). The “interpreted as” row shows exactly which reading happened.

Developer & Tech

Unix Timestamp Converter

What this does

Convert Unix seconds, milliseconds or ISO 8601 strings into every other format automatically, with relative-time context included.

Enter your details

Runs in your browser

How to use it

Using the unix timestamp converter

01
Paste any timestamp form
Epoch seconds from a database, milliseconds from JavaScript, or an ISO string from an API log.
02
Read all representations
Seconds, milliseconds and ISO UTC appear together so cross-format debugging takes one glance.
03
Sanity-check relatively
The “X ago” row instantly exposes unit mistakes; a wrong guess reads as millennia away.

Good to know

Why magnitude auto-detection works

Seconds since 1970 crossed 1e11 only around the year 5138; milliseconds passed that mark in 1973. The threshold cleanly separates the two formats for any realistic timestamp, making conversion frictionless without a mode switch.

Timestamp bugs worth avoiding

Storing seconds in a millisecond column (or vice versa) silently corrupts sorting
new Date(string) parses differently across engines for non-ISO strings
Always serialize UTC; convert to local time only at display
JavaScript Date lacks minutes-level precision beyond ±8.64e15 ms

How it's calculated

The math behind this calculator

ms > 1e11 → read as milliseconds; else seconds; then derive all other representations

Purely numeric input auto-detects its unit by magnitude: absolute values above one hundred billion can only be milliseconds, anything smaller reads as seconds. Non-numeric input attempts ISO 8601 parsing through the standard Date constructor.

Once parsed, the instant renders simultaneously as Unix seconds, Unix milliseconds and an ISO UTC string, plus a coarse relative description (“in 3 days”, “2 hours ago”) computed against your current clock for instant sanity checking.

Assumptions & limitations

All output is UTC; local time zones affect display only.
Magnitude detection assumes modern epochs; dates before 1973 in seconds look like milliseconds edge cases.
Leap seconds are ignored, per Unix-time convention.

Worked example

The round number 1,700,000,000 seconds lands on 2023-11-14T22:13:20Z; late evening UTC, handy for spotting off-by-a-day bugs.

FAQ

Frequently asked questions

How does it tell seconds from milliseconds?: Pure numbers with absolute value over 100 billion are treated as milliseconds; otherwise seconds. Dates before 1973 are the rare ambiguity zone.
Is the ISO output always UTC?: Yes; toISOString() is defined as UTC with Z suffix. Convert to your time zone at presentation time, never before storage.
Can I convert negative timestamps?: Yes; pre-1970 dates work as negative seconds/milliseconds throughout.
Why does my relative time say “55 years ago” for a small number?: You likely pasted milliseconds into what was interpreted as seconds (or vice versa). The “interpreted as” row shows exactly which reading happened.

Keep exploring

Using the unix timestamp converter

Paste any timestamp form

Read all representations

Sanity-check relatively