How this calculator works
Unix time counts the seconds since 00:00:00 UTC on 1 January 1970, a moment called the Unix epoch. It is a single signed number, so it has no time zone, no daylight saving and no calendar structure until it is read as a date. Negative values count backwards into 1969 and earlier. Because it is defined in UTC, two computers in different time zones agree on the same timestamp for the same instant. A timestamp is commonly written in seconds, milliseconds, microseconds or nanoseconds depending on the system, and the unit must be known to read it correctly.
Formula
seconds = milliseconds / 1,000, microseconds = 1,000,000 x seconds, and nanoseconds = 1,000,000,000 x seconds. The date is obtained by adding the seconds to 1970-01-01T00:00:00Z. To go the other way, the date is first read as a UTC calendar date and the difference from the epoch is expressed in seconds.
Worked example
0 is Thursday, 1 January 1970 at 00:00:00 UTC. 1767225600 is Thursday, 1 January 2026 at 00:00:00 UTC, and its ISO form is 2026-01-01T00:00:00.000Z. The same instant in milliseconds is 1,767,225,600,000.
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Frequently asked questions
Does Unix time count leap seconds?
No. Unix time counts the elapsed seconds of the UTC day clock as if every day had 86,400 seconds. Leap seconds are stepped around, which is why Unix time can differ from a continuous atomic timescale.
How do I tell whether a timestamp is in seconds or milliseconds?
A current timestamp in seconds has ten digits, while the same instant in milliseconds has thirteen. A value with thirteen digits is almost always milliseconds; the unit selector makes the choice explicit.
Why does my local date differ from the UTC result?
The page works in UTC. A time zone can be ahead of or behind UTC, so the local calendar date can be the day before or after the UTC date near midnight.
What range of dates is supported?
The page accepts instants that JavaScript can represent, about 273,790 years before or after 1970. Input dates are limited to four-digit years from 1000 to 9999.