What it does
A Unix timestamp is a single number that identifies an instant. This tool converts one into a human-readable date and back again. It also exposes the same instant as seconds, milliseconds, an ISO 8601 string, a UTC string, and a local date with the device’s timezone.
Timestamps are compact, sortable, and independent of language and timezone, which is why they appear in logs, APIs, databases, cookies, and filenames. They are also easy to misread: a ten-digit number is usually seconds, a thirteen-digit number is usually milliseconds, and a value that looks plausible in one unit can point to a completely different year in another.
Seconds, milliseconds, and more
The Unix epoch is 1 January 1970 at 00:00:00 UTC. A seconds timestamp counts from there. A milliseconds timestamp counts the same interval in thousandths of a second, so it has three more digits. Microseconds and nanoseconds extend the same pattern. The tool can read all four and will label the unit it used.
Auto-detection is a convenience, not a standard. It uses digit count: up to ten digits is seconds, eleven to thirteen is milliseconds, fourteen to sixteen is microseconds, and longer values are nanoseconds. That covers ordinary dates around the present. If you are working with a far-future date, a deliberately small value, or an API with a fixed unit, choose the unit explicitly and the guess disappears.
Timezones and the epoch
The epoch itself is defined in UTC. UTC is not a timezone with daylight saving; it is the reference clock. Local time is what your operating system shows after applying its timezone rules. The tool prints both. A timestamp does not contain a timezone — it identifies an instant — so there is no “correct” local hour without knowing where the reader is.
Dates before 1970 are negative timestamps. JavaScript dates can represent a very large range, but not every date string is unambiguous. An ISO value with Z or an offset is the safest input. A bare date such as 2026-10-27 is treated as UTC midnight by the ECMAScript date parser, while a bare date-time without an offset is usually local. The output shows the resulting UTC and local forms, so you can catch an unintended reading.
Examples
Before
1700000000After
Unix seconds: 1700000000
Unix milliseconds: 1700000000000
ISO 8601 (UTC): 2023-11-14T22:13:20.000Z
UTC: Tue, 14 Nov 2023 22:13:20 GMT
Local: (your device timezone)
Weekday (UTC): TuesdayThe timestamp is 14 November 2023 at 22:13:20 UTC. The local line depends on the device viewing the page, which is why it is shown as a placeholder here rather than pretending there is one universal local time. The weekday is derived from the UTC date.
Before
2000-01-01T00:00:00ZAfter
Unix seconds: 946684800
Unix milliseconds: 946684800000
ISO 8601 (UTC): 2000-01-01T00:00:00.000Z
UTC: Sat, 01 Jan 2000 00:00:00 GMT
Local: (your device timezone)
Weekday (UTC): SaturdayMidnight on 1 January 2000 UTC is 946,684,800 seconds after the epoch. The same instant is 946,684,800,000 milliseconds. If the input had no trailing Z, the browser would interpret it in the local zone, and the seconds value could move by hours.
Limits worth knowing
- Precision stops at milliseconds. JavaScript’s Date stores an integer number of milliseconds. Microsecond and nanosecond inputs are converted to the nearest millisecond; the smaller digits are not preserved.
- Leap seconds are not modeled. The Date timeline assumes every day has 86,400 seconds. A timestamp at a leap-second boundary can differ by one second from a specialized time scale.
- Auto-detection can guess wrong. A 13-digit seconds value is unusual, but it is not impossible. The unit selector exists because digit count is a heuristic, not a property of the number.
- Local means this device. The local output uses the browser’s timezone and daylight-saving rules. A colleague in another zone will see a different local hour for the same instant.
- It is not a calendar library. It does not handle named timezones such as “Europe/Berlin” as input, recurring events, or calendar arithmetic. Use a dedicated date library when you need those rules.
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