What the Unix Epoch is
The Unix timestamp, also called Unix Epoch or Unix time, is the number of seconds that have passed since January 1, 1970, at midnight UTC. That reference point was chosen by the creators of the Unix operating system, and it became the standard for representing dates and times in most programming languages, databases and APIs.
Seconds vs milliseconds
Some systems, like traditional Unix and PHP, work with timestamps in seconds, while others, like JavaScript, work natively in milliseconds. A quick way to tell them apart: for current years, a timestamp in seconds has 10 digits, and the same moment in milliseconds has 13 digits, three more, since it is the value in seconds multiplied by 1,000.
Why systems use timestamps instead of formatted dates
Storing dates as a timestamp, a single number, instead of a formatted date string has practical advantages: it is easier to store, compare, add or subtract time intervals, and it avoids ambiguity about the format used, such as day/month/year versus month/day/year. The timestamp only needs to be converted into a readable format when the information is displayed to a person.
Timestamps and time zones
An important detail is that a Unix timestamp by itself carries no time zone information: it is always an absolute value relative to UTC. The "local date" shown when converting a timestamp depends on the time zone of the device or browser doing the conversion, so the same timestamp can correspond to different times of day depending on where the person is.
ISO 8601 format
Besides the Unix timestamp, this calculator also shows the date in ISO 8601 format, such as "2025-01-01T00:00:00.000Z", an international standard for representing dates and times unambiguously, widely used in APIs and log files, since it orders the components from largest to smallest, year, month, day, time, and clearly marks the time as UTC with the trailing "Z".