Current epoch
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Starting the clock…
Reads the date above, and shows the pasted timestamp as a local time.
How this one works
A Unix timestamp counts the time since 1 January 1970 UTC. Systems disagree on the unit: databases and log lines usually use seconds, JavaScript and Java use milliseconds, and Postgres and tracing tools often use microseconds. Paste a number and the unit is guessed from its size: a present-day value has 10 digits in seconds, 13 in milliseconds and 16 in microseconds, with the picker there to overrule it.
Going the other way, the date you pick is a wall-clock reading, so it only becomes an instant once a zone is attached. That is done with your browser's own time zone database, which is why daylight saving comes out right, including the hour that never happens in spring and the one that happens twice in autumn.
Questions
Is my timestamp sent anywhere?
No. The conversion is JavaScript running in this tab, using the time zone data already in your browser. Nothing is uploaded, and the page makes no network requests at all.
Why does the same number give a different date on another site?
Almost always the unit. 1789000000 read as seconds is 2026; read as milliseconds it is 1970. Set the unit explicitly rather than relying on detection when the value is unusual.
Do negative timestamps work?
Yes. A negative value counts backwards from 1970, so -14182940 is the Apollo 11 landing in July 1969. Dates run from the year -271821 to 275760, the range a JavaScript date can hold.
What about leap seconds?
Unix time ignores them, and so does this page. Every day is treated as exactly 86,400 seconds, which is why a timestamp never lands on a leap second, the same assumption your database and programming language make.