Unix Timestamp FAQ
Quick answers to the most common Unix timestamp questions — units, timezone handling, the Year 2038 problem, language-specific conversions, and more.
Frequently Asked Questions
- What is a Unix timestamp?
- A Unix timestamp is the number of seconds that have elapsed since the Unix epoch — January 1, 1970 at 00:00:00 UTC. It is a language-neutral, timezone-independent integer that uniquely identifies any moment in time.
- What is the Unix epoch?
- The Unix epoch is January 1, 1970 at 00:00:00 UTC. All Unix timestamps are measured forward — or backward for negative values — from this fixed reference point. The choice of 1970 was practical: it was a recent, round year when Unix was being designed at Bell Labs.
- What is the difference between a 10-digit and a 13-digit timestamp?
- A 10-digit timestamp (e.g. 1700000000) represents seconds since the epoch. A 13-digit timestamp (e.g. 1700000000000) represents milliseconds. JavaScript's Date.now() returns milliseconds; most Unix system calls and languages like Python, Go, and PHP return seconds by default.
- Why does JavaScript use milliseconds instead of seconds?
- JavaScript's Date API was designed to match the resolution of system timers available in 1995. Using milliseconds avoids precision loss for sub-second durations. When working with server-generated Unix timestamps in seconds, multiply by 1000 before passing to new Date().
- How do I convert a Unix timestamp to a readable date in JavaScript?
- For a seconds timestamp: new Date(1700000000 * 1000).toISOString(). For a milliseconds timestamp: new Date(1700000000000).toISOString(). Both produce ISO 8601 strings like '2023-11-15T06:13:20.000Z'. Use toLocaleString() with a timeZone option for localized display.
- Are Unix timestamps timezone-aware?
- No. A Unix timestamp always represents a single moment in UTC. Timezone information is only applied when formatting the timestamp for display. Two devices in different timezones will produce the same Unix timestamp for the same instant.
- What is the Year 2038 problem?
- Systems that store Unix timestamps as 32-bit signed integers can only represent values up to 2,147,483,647, which corresponds to January 19, 2038 at 03:14:07 UTC. After that moment the integer overflows to a large negative number. Modern 64-bit systems are not affected, but embedded systems and legacy databases may still be at risk.
- What is a negative Unix timestamp?
- A negative Unix timestamp represents a moment before the Unix epoch. For example, -86400 is December 31, 1969 at 00:00:00 UTC. Most modern date APIs handle negative timestamps correctly, but some older systems and databases do not support them.
- How do I get a Unix timestamp in Python?
- Use import time; int(time.time()) for seconds. For an explicit UTC datetime: import datetime; int(datetime.datetime.now(datetime.timezone.utc).timestamp()). For milliseconds: int(time.time() * 1000).
- How precise is a Unix timestamp?
- A standard Unix timestamp has one-second precision. For sub-second precision, millisecond timestamps (13 digits) are common in web APIs and JavaScript. Some systems use microseconds (16 digits) or nanoseconds. The appropriate unit depends on the precision your application actually needs.