Unix timestamp converter

What is a Unix timestamp?

A Unix timestamp counts the seconds since 00:00:00 UTC on 1 January 1970, the Unix epoch. It has no time zone: 1790853909 is the same instant in Tokyo and in Lima. Time zones only apply when you turn it into a calendar date for a person to read.

Systems disagree on the unit. C, Go, PostgreSQL and most shells use seconds. JavaScript, Java and many JSON APIs use milliseconds. Python’s time.time_ns(), Postgres internals and tracing tools use microseconds or nanoseconds. You can usually tell by the length: for dates in this century, 10 digits is seconds, 13 is milliseconds and 16 is microseconds.

Unix time ignores leap seconds. Every day is exactly 86,400 seconds, so timestamps stay simple to compare and subtract.

Common mistakes

1. 01Passing seconds where milliseconds are expectednew Date(1790853909) in JavaScript gives 21 January 1970. Multiply by 1000 first, or check the digit count.
2. 02Treating the timestamp as local timeA timestamp is always UTC. Converting it “from” a time zone shifts it by the offset and produces a wrong instant.
3. 03Storing it in a 32-bit integerSigned 32-bit seconds run out on 19 January 2038. Use a 64-bit column, such as bigint in SQL.
4. 04Rounding milliseconds with integer division in the wrong directionMath.floor and integer division differ for negative values. Use floor division when converting dates before 1970.