What is a millisecond Unix timestamp?

It is the number of milliseconds elapsed since 1970-01-01T00:00:00Z. This is exactly what JavaScript's Date.now() and Date.getTime() return, and a present-day value is about 13 digits long.

How is it different from a seconds timestamp?

A millisecond timestamp is 1000 times larger than the same instant in seconds, so it has ~13 digits instead of ~10. Divide by 1000 to get seconds, multiply by 1000 to go the other way.

Why does my date come out wrong by decades?

You probably mixed up units. Treating a seconds value (10 digits) as milliseconds, or vice versa, shifts the result by roughly fifty years. Check the digit count first.

Does this show UTC or local time?

Both. The epoch value is timezone-independent; the tool renders the instant in UTC and also in your browser's local timezone.

Is anything uploaded?

No. The conversion runs entirely in your browser with the built-in Date API. Nothing leaves your device.

Unix Timestamp (Milliseconds) Converter

A Unix timestamp in milliseconds is the number of milliseconds since 1970-01-01T00:00:00Z. It is the native time unit in JavaScript: Date.now() and Date.prototype.getTime() both return milliseconds, which is why millisecond timestamps show up constantly in web logs, APIs and front-end code. This converter decodes those values into readable dates and encodes dates back into milliseconds, all locally.

How it works

JavaScript’s Date constructor already takes milliseconds, so decoding is direct:

const date = new Date(epochMilliseconds);
date.toISOString(); // UTC ISO-8601

To encode a date-time back to milliseconds, the tool parses the string and reads getTime() directly — no scaling needed.

A present-day millisecond timestamp is about 13 digits (for example 1700000000000). If your value is only ~10 digits it is in seconds, and you should use the seconds converter so the date is not off by ~50 years.

Tips and notes

Sub-second precision (the final three digits) is preserved in the millisecond field.
Negative values represent dates before the 1970 epoch.
Store and transmit the UTC ISO-8601 form; use the local form only for display.