Unix time explained, and a converter that goes both ways

A Unix timestamp is a single integer: the number of seconds since midnight UTC on 1 January 1970. No time zone, no format, no locale. That is the entire point. Every server, database and log file can agree on one number, and the messy business of turning it into something a human reads happens at the last possible moment.

This tool converts a timestamp into eight readable forms, converts a date back into a timestamp, and shows a live clock ticking the current epoch second so you can grab it with one click.

Reading a timestamp by its length

Different systems emit different precisions. Digit count tells you which one you are holding, and getting it wrong throws your date off by roughly fifty years.

The tool auto detects thirteen digit input as milliseconds regardless of the unit toggle, because that is the overwhelmingly common mistake. For anything else, set the toggle explicitly.

What the epoch actually is

The 1 January 1970 zero point was chosen by the team building Unix at Bell Labs in the early 1970s. It was recent, round, and left plenty of headroom in a 32 bit signed integer. Timestamps before it are negative, which is legal and works fine in most modern languages.

Leap seconds, and why Unix time ignores them

Unix time defines every day as exactly 86,400 seconds. Actual astronomical time does not cooperate, because the Earth’s rotation is slowly and unevenly slowing down. The International Earth Rotation Service has inserted 27 leap seconds since 1972 to keep UTC aligned with solar time.

Unix time simply skips them. When a leap second happens, the timestamp repeats a value or the clock is smeared across a window. Google and Amazon both use leap smearing, spreading the extra second across 24 hours so no clock ever jumps. What this means practically: Unix time is not a true count of elapsed SI seconds since 1970. It is off by 27 seconds and always will be. For anything short of precision astronomy or high frequency trading, that is irrelevant. In 2022 the international bodies agreed to stop adding leap seconds by 2035.

Generating timestamps in code

Output formats this tool produces

Local time and relative time are computed against your device clock and time zone, which the browser reports through Intl.DateTimeFormat().resolvedOptions().timeZone. Everything else is time zone independent by definition.

A note on storing timestamps

Store the integer, not the formatted string. Formatted dates carry a locale assumption, sort incorrectly as text, and lose information about the time zone they were rendered in. If you need to preserve the user’s intended wall clock time, which matters for calendar events and scheduled jobs, store the local date, the local time and the IANA time zone identifier separately. A timestamp alone cannot represent that a meeting is at 9am in Berlin, because the offset changes twice a year.

Specifications and further reading

• POSIX seconds since the Epoch is the formal definition, including the explicit statement that leap seconds are excluded.
• RFC 3339 defines the timestamp string format used across internet protocols.
• IANA Time Zone Database is the source of truth for every offset and daylight saving rule on Earth. Updated several times a year as governments change their minds.
• MDN Date reference covers the JavaScript API and its many sharp edges.
• Temporal.Instant is the modern JavaScript replacement, with nanosecond precision and no ambiguity.
• Year 2038 problem for the full history of the 32 bit overflow and current mitigation status.

Everyday uses

Reading a raw log line, debugging a JWT expiry claim, checking when a cache entry was written, setting a cookie or token lifetime, decoding a database column that came back as a bare integer, comparing event ordering across servers in different regions, and sanity checking that a scheduled job fired when you thought it did.