What a Unix timestamp is
A Unix timestamp (also called epoch time or POSIX time) is the number of seconds that have elapsed since 00:00:00 UTC on January 1, 1970, not counting leap seconds. It is a single number with no time zone attached, which is exactly why databases, log files, APIs and operating systems use it: the same instant has the same timestamp in New York, London and Tokyo. Converting it to something readable means choosing a time zone. Timestamp 1700000000, for example, is 22:13:20 UTC on November 14, 2023, which is 5:13 p.m. Eastern Standard Time the same day and 2:13 p.m. Pacific.
Two precisions are common. Seconds give a 10-digit number today (1,700,000,000 and counting); milliseconds give 13 digits. JavaScript's Date.now() and Java's System.currentTimeMillis() return milliseconds, while Python's time.time(), PHP's time() and the Unix date +%s command return seconds. This converter treats any value of 100,000,000,000 or more as milliseconds and anything smaller as seconds. Mixing them up is the classic bug: a seconds value read as milliseconds lands in January 1970, and a milliseconds value read as seconds lands tens of thousands of years in the future.
Time zones and daylight saving time
The result table shows the instant in UTC, in your browser's local zone, and in the four main US zones using the IANA names America/New_York, America/Chicago, America/Denver and America/Los_Angeles. Those names are used rather than fixed offsets like UTC−5 because the offsets change twice a year: Eastern time is UTC−5 (EST) in winter and UTC−4 (EDT) from the second Sunday in March to the first Sunday in November. Your browser's built-in time zone database handles the switch, and the suffix in each row (EST or EDT, and so on) tells you which applied. Arizona and Hawaii do not observe daylight saving time; use the Mountain row for Arizona in winter only.
To go the other way, enter a date and time and choose which zone it should be read in. A meeting at 9:00 a.m. Pacific and one at 9:00 a.m. Eastern are three hours apart and produce different timestamps. The tool finds the correct offset for that specific date, including the daylight saving rule in effect.
Formats and limits
ISO 8601 is the standard text form: 2023-11-14T22:13:20.000Z, where the trailing Z means UTC and a suffix like −05:00 would denote an offset. RFC 2822, the older email-header style (Tue, 14 Nov 2023 22:13:20 GMT), is also shown. Store timestamps or ISO strings in UTC and convert for display; storing local times without a zone is how appointments end up an hour off after the clocks change.
Systems that keep the timestamp in a signed 32-bit integer overflow at 03:14:07 UTC on January 19, 2038 (timestamp 2,147,483,647), the so-called Year 2038 problem. Modern 64-bit systems and JavaScript are not affected, but older embedded devices and some database columns are. Negative timestamps represent dates before 1970 and are supported here, though not by every API.