It was a round, recent date when Unix was being developed around 1970, convenient as a starting point. The choice stuck and became a near-universal standard.

Does a timestamp include a timezone?

No — it is always counted in UTC. You apply a timezone offset only when converting the number to a human-readable local date and time.

How do I convert one by hand?

It's tedious by hand — most people paste the number into a converter. Programmatically, most languages have a built-in function (for example new Date(seconds * 1000) in JavaScript).

Do leap seconds affect it?

Unix time ignores leap seconds by design — it assumes every day is exactly 86,400 seconds. This keeps arithmetic simple, at the cost of not being a perfect count of physical seconds.

What Is a Unix Timestamp?

Published 2/17/2026 · 2 min read · Developer tools

Daniel Okonkwo — Front-end developer and tech writer at OneKitly

Web performance · File formats

Checked against 2 sources

View profile →

In short

A Unix timestamp is a single number: the count of seconds elapsed since midnight UTC on January 1, 1970, called the epoch. For example 1735689600 is January 1, 2025. Computers store time this way because one integer is easy to compare, sort and do math on, and it is timezone-independent — you convert to a local date only for display.

A Unix timestamp counts the seconds since January 1, 1970 UTC. Learn why computers use it, how to convert it, and the 2038 problem.

The epoch: a common zero point

Every clock needs a starting point. Unix systems chose midnight UTC on January 1, 1970, and count seconds forward from there. Times before it are negative numbers. Because the whole computing world shares this zero, systems can exchange a timestamp and agree on the exact instant without ambiguity.

Seconds vs milliseconds

Classic Unix time is in seconds, but many systems (notably JavaScript) use milliseconds — 1000 times larger. If a timestamp looks about 1,000× too big, it is probably in milliseconds; divide by 1000 to get seconds before converting. A quick sanity check: a 10-digit number is seconds today, a 13-digit one is milliseconds.

The year 2038 problem

Older systems store the timestamp in a signed 32-bit integer, which overflows on January 19, 2038, when the count exceeds about 2.1 billion. Such systems would wrap around to a negative number and misread the date. The fix is a 64-bit integer, which pushes the limit billions of years away; modern platforms have largely moved to it.

Unix Timestamp ConverterConvert a Unix timestamp into a readable UTC date and time.Try the tool →

Frequently asked questions

Why 1970?: It was a round, recent date when Unix was being developed around 1970, convenient as a starting point. The choice stuck and became a near-universal standard.
Does a timestamp include a timezone?: No — it is always counted in UTC. You apply a timezone offset only when converting the number to a human-readable local date and time.
How do I convert one by hand?: It's tedious by hand — most people paste the number into a converter. Programmatically, most languages have a built-in function (for example new Date(seconds * 1000) in JavaScript).
Do leap seconds affect it?: Unix time ignores leap seconds by design — it assumes every day is exactly 86,400 seconds. This keeps arithmetic simple, at the cost of not being a perfect count of physical seconds.

Articles you may find interesting

All guides →

ExplainerHow Unix File Permissions Work: Reading 755 Without GuessingRead is 4, write is 2, execute is 1, and each of the three digits describes a different party. The part most explanations get wrong is what the execute bit does on a directory — it grants traversal, not the right to run anything.ExplainerBig O Notation for Beginners: O(1), O(n), O(n squared), and O(log n) ExplainedBig O describes how an algorithm's work grows as input grows. Here is what O(1), O(n), O(n squared), and O(log n) mean and why the difference matters.ExplainerWhat Is UTF-8 and Unicode? Code Points, Byte Encoding, and Why UTF-8 WonUnicode assigns every character a code point; UTF-8 encodes those code points in one to four bytes. Here is how it works and why it beat the alternatives.ExplainerPassword Entropy Explained: Bits, Length, and How Long a Password Takes to CrackWhat password entropy really measures, why length beats complexity, and how bits of entropy translate into a realistic crack time.ExplainerScreen Resolutions Explained: 1080p, 1440p, 4KWhat 1080p, 1440p, and 4K really mean. Learn how resolution counts pixels, why total pixels grow so fast, and how aspect ratio ties width to height.ExplainerWhat Is Bitrate? Bits per Second, CBR vs VBRBitrate is how many bits of data a video or audio stream uses each second. Learn how it sets quality and file size, and the difference between constant and variable bitrate.

Sources

Spotted a mistake in this article?