How this calculator works
A Unix timestamp (also called epoch time or POSIX time) is a single number representing a point in time: the number of seconds that have elapsed since the Unix epoch, 00:00:00 Coordinated Universal Time (UTC) on 1 January 1970, not counting leap seconds. Many systems store timestamps in milliseconds instead of seconds, so this converter auto-detects which unit a pasted number is in and shows both the human-readable UTC date and time and the equivalent ISO 8601 string.
For the reverse direction, enter a date and a time and the calculator treats it as a UTC instant and returns the matching timestamp in both seconds and milliseconds. All conversions are done with UTC calendar math, so there is no dependence on the visiting device's clock, time zone, or daylight-saving rules.
How the conversion works
timestamp (seconds) = (UTC instant − 1970-01-01T00:00:00Z) ÷ 1 secondmilliseconds = seconds × 1,000Unit auto-detection: values with an absolute size under 100,000,000,000 (1e11) are treated as seconds; values at or above that are treated as milliseconds. A seconds-based timestamp only reaches 1e11 around the year 5138, while a milliseconds-based timestamp for any date after 1973 is already above it, so ordinary present-day timestamps never fall in the ambiguous zone.
Worked example: decoding 1700000000000
- The pasted value is 1,700,000,000,000 — 13 digits, well above the 1e11 auto-detection threshold, so it is treated as milliseconds.
- Dividing by 1,000 gives 1,700,000,000 seconds since the epoch.
- Converting that many seconds forward from 1970-01-01T00:00:00Z lands on 2023-11-14 22:13:20 UTC.
- The equivalent ISO 8601 string is 2023-11-14T22:13:20.000Z.
Frequently asked questions
Why do some timestamps have 10 digits and others 13?
10-digit timestamps are in seconds (typical of Unix/Linux systems, PHP, and most APIs); 13-digit timestamps are in milliseconds (typical of JavaScript's Date.now() and many web APIs). Both count from the same 1970-01-01 epoch — only the unit differs.
Does a Unix timestamp account for time zones?
No. A Unix timestamp always refers to a single, unambiguous instant in UTC. Any time zone is applied only when you convert that instant into a local calendar date and clock time — the timestamp number itself never changes.
What happens with dates before 1970?
They are represented as negative timestamps. For example, -86,400 seconds is exactly one day before the epoch: 1969-12-31 00:00:00 UTC. This converter handles negative values the same way as positive ones.
Do Unix timestamps count leap seconds?
No — Unix time ignores leap seconds by definition, effectively repeating or skipping a second on the rare days a leap second is inserted. This makes Unix time simple to do arithmetic with, at the cost of not exactly tracking true elapsed atomic time over long periods.
Why is 1,000,000,000 seconds nicknamed the 'billennium'?
It was the moment, on September 9, 2001 at 01:46:40 UTC, when the Unix timestamp counter first reached ten digits starting with a 1 followed by nine zeros — a round-number milestone some engineers marked much like New Year celebrations.