UnixEpoch

The Unix epoch, commonly referred to as Unix time, is the reference point used by Unix and most Unix-like systems to measure time. It is defined as 00:00:00 Coordinated Universal Time (UTC) on January 1, 1970. Time is represented as the number of seconds that have elapsed since that moment, with the count advancing in whole seconds and not accounting for leap seconds.

In practice, most systems store this value as a time representation called time_t, which is an integer count of seconds since the epoch. The size of time_t varies by platform: some systems use a 32-bit integer, while others use a 64-bit integer. The 32-bit form faces the year 2038 problem, when the maximum positive value is reached and time representations may wrap around. Modern systems increasingly adopt 64-bit time_t to extend the usable range far beyond 2038. Some programming environments use alternative units, such as milliseconds since the epoch (for example, many languages and JavaScript), rather than seconds.

The epoch is used to timestamp a wide range of data, including files, processes, logs, and events. Converting an epoch value to a human-readable date requires applying a time zone and locale; displaying in UTC avoids local time adjustments. Many libraries provide functions to format, parse, and manipulate epoch times, enabling interoperability across different systems and programming languages.

Limitations of the Unix epoch include its treatment of leap seconds, which are not represented in the basic time count. Leap seconds are occasionally added to UTC, but Unix time advances by a constant number of seconds, with some systems handling corrections in different ways. Despite these issues, the epoch remains a foundational time standard in computing and data exchange.