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What is the Unix epoch? Why computers count seconds from January 1, 1970

Unix timestamps are the simplest way to store and compare wall-clock points in time across systems. They're timezone-agnostic and supported by every programming language and database, but wall clocks can jump forward or backward when they are synchronized. Use a monotonic clock for elapsed-time measurements, then display Unix timestamps in the user's local timezone.

Timestamp 0 in Unix time is 00:00:00 UTC on January 1, 1970, the Unix epoch. Every Unix timestamp is the number of seconds that have elapsed since that moment, not counting leap seconds. It's a beautifully simple system: one integer represents any point in time, timezone-independent, sortable, and mathematically trivial to compare.

By TechCompare · Updated

Unix timestamp
0
Seconds since epoch
ISO 8601
1970-01-01T00:00:00.000Z
Thu, 01 Jan 1970 00:00:00 GMT
Category
Fundamentals
Timestamp knowledge base
Try the live Unix Timestamp Converter. Paste any timestamp or date and convert it instantly.
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How this is calculated

Unix time counts seconds from 00:00:00 UTC on January 1, 1970. The published definition sets that epoch but doesn't establish that it was chosen to fit a 32-bit signed integer. The 2038 limit is a later consequence of storing Unix seconds in a signed 32-bit value, which overflows at 2,147,483,647 seconds after the epoch. Modern systems use wider timestamps to avoid that limit.

Verdict

Unix time counts seconds from January 1, 1970, and POSIX defines days as exactly 86,400 seconds while ignoring leap seconds. The documented definition doesn't establish that the epoch was selected to fit 32-bit timestamps. A signed 32-bit count reaches its limit on January 19, 2038, which is a consequence of that storage width, not a documented reason for choosing the epoch. Wider timestamp types avoid the limit. Store an unambiguous timestamp and convert it for display in the user's timezone.

More Timestamp scenarios

Year 2038 problem
On January 19, 2038, at 03:14:07 UTC, 32-bit signed Unix timestamps will overflow and wrap around to negative values, representing December 13, 1901.
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ISO 8601 vs Unix timestamps
ISO 8601 dates (like 2026-06-03T18:30:00Z) are human-readable and self-describing.
View details ➜

Related guides

ISO 8601 vs Unix timestamps
Why Unix epoch timestamps pair with ISO 8601 in APIs.
View guide ➜
Cron vs systemd timers scheduling
How scheduled jobs count from this Unix epoch.
View guide ➜

Frequently asked questions

Why is the Unix epoch January 1, 1970?
Unix time is defined to start at 00:00:00 UTC on January 1, 1970. The published definition doesn't establish why that date was selected or say it was chosen to fit a 32-bit integer. January 1, 1970, was a Thursday.
Does Unix time count leap seconds?
No, by convention Unix time ignores leap seconds. When a leap second is added to UTC, Unix timestamps continue counting as if the leap second didn't exist. POSIX defines the day as exactly 86,400 seconds regardless of actual UTC time. This keeps math simple at the cost of small drift over decades.
How do I convert a Unix timestamp to a human-readable date?
Use a built-in function in your language. JavaScript: `new Date(timestamp * 1000).toISOString()`. Python: `datetime.utcfromtimestamp(timestamp).isoformat()`. Go: `time.Unix(timestamp, 0).UTC().Format(time.RFC3339)`. Most languages also have a timezone-aware variant. Always store UTC in your database and convert to local time only at the presentation layer.

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