Unix Time Converter

Computers agree on time by counting from one agreed moment. That moment is the Unix epoch, 1970-01-01T00:00:00Z, and a Unix timestamp is the number of seconds since then. Paste one and the tool renders it four ways, pick a date and it gives the timestamp back, and a clock at the top shows the current epoch in seconds and milliseconds, updated every second.

What it computes

The input must be a whole number, negative for instants before 1970. The tool decides the unit, then builds a JavaScript Date, which counts milliseconds since the epoch:

unit          = milliseconds if 12 or more digits, else seconds
ms            = seconds × 1000, or the value itself
UTC           = HTTP-date format, e.g. Tue, 14 Nov 2023 22:13:20 GMT
ISO 8601      = YYYY-MM-DDTHH:mm:ss.sssZ, always in UTC
Local         = full date and long time, your locale and time zone
Relative      = floor(|ms − now| ÷ unit size), largest unit that fits
date → epoch  = floor(picked local time in ms ÷ 1000)

The s and ms buttons override the automatic guess. Relative time uses fixed unit sizes, a 365-day year and a 30-day month, then days, hours, minutes and seconds, and it reports only the largest whole unit. As in POSIX time, every day is exactly 86,400 seconds, so leap seconds do not exist in this arithmetic.

A worked example

The default timestamp is 1700000000.

1. It has 10 digits, fewer than 12, so it is read as seconds: 1700000000 × 1000 = 1700000000000 ms.
2. At 86,400 seconds a day that is 19,675 whole days after 1 January 1970, which lands on 14 November 2023, plus 80,000 seconds, which is 22:13:20.
3. The four rows render that instant:

The local row depends on your device: the same instant is 3:43:20 AM on 15 November in India. The relative row depends on the clock: in October 2026 the gap is about 1,072 days, two whole 365-day years, and from 13 November 2026 it reads 3 years ago.

The reverse field starts at 2023-11-14T22:13, read as local time. In a UTC zone that gives 1699999980, 20 seconds before the default timestamp because the picker drops seconds. In New York, five hours behind UTC, the same wall-clock time is 1700017980.

Assumptions and limits

• Whole numbers only. A fractional timestamp such as 1700000000.5, or one with separators, is rejected.
• The 12-digit rule. Seconds values of 12 or more digits, after the year 5138, and millisecond values of 11 or fewer, before 3 March 1973, are misread unless you force the unit. Microsecond and nanosecond timestamps, 16 and 19 digits today, need dividing by 1,000 or 1,000,000 first.
• No leap seconds. The 27 leap seconds added since 1972 are not counted, so the value is not an exact tally of elapsed SI seconds.
• Range. JavaScript dates reach about 273,000 years either side of 1970; beyond that nothing is shown.
• Local means this device. The local row and the date picker use your browser’s time zone and daylight-saving rules.

How to use the result

• Spot the 1,000-times bug. A date in January 1970 or tens of thousands of years ahead means the value was read in the wrong unit.
• Check token and cache expiry. JWT exp, iat and nbf claims are seconds. Paste one to see when a token expires and compare it with the live epoch.
• Store UTC, display local. Keep the epoch value or an ISO 8601 string ending in Z in databases, logs and APIs, and convert to local time only for people.
• Reuse the UTC row in HTTP headers. It is the date format of headers such as Last-Modified and Expires.
• Mind 2038. A field that holds seconds in a signed 32-bit integer overflows after 2147483647, which is 2038-01-19T03:14:07Z.