Unix time, or POSIX time, is a system for describing instants in time, defined as the number of seconds that have elapsed since midnight Coordinated Universal Time (UTC), January 1, 1970, not counting leap seconds. It is used widely in Unix-like and many other operating systems and file formats. It is neither a linear representation of time nor a true representation of UTC. Unix time may be checked on some Unix systems by typing date +%s on the command line.

Two layers of encoding make up Unix time. These can usefully be separated. The first layer encodes a point in time as a scalar real number, and the second encodes that number as a sequence of bits or in another form.^[specify]

As is standard with UTC, this article labels days using the Gregorian calendar, and counts times within each day in hours, minutes, and seconds. Some of the examples also show International Atomic Time (TAI), another time scheme, which uses the same seconds and is displayed in the same format as UTC, but in which every day is exactly 86 400 s long, gradually losing synchronization with the Earth's rotation at a rate of roughly one second per year.

Unix (officially trademarked as UNIX, sometimes also written as Unix) is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna. The Unix operating system was first developed in assembly language, but by 1973 had been almost entirely recoded in C, greatly facilitating its further development and porting to other hardware. Today's Unix system evolution is split into various branches, developed over time by AT&T as well as various commercial vendors, universities (such as University of California, Berkeley's BSD), and non-profit organizations.

The Open Group, an industry standards consortium, owns the UNIX trademark. Only systems fully compliant with and certified according to the Single UNIX Specification are qualified to use the trademark; others might be called Unix system-like or Unix-like, although the Open Group disapproves of this term. However, the term Unix is often used informally to denote any operating system that closely resembles the trademarked system.

Time is the indefinite continued progress of existence and events that occur in apparently irreversible succession from the past through the present to the future. Time is a component quantity of many measurements used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience. The temporal position of events with respect to the transitory present is continually changing; events happen, then are located further and further in the past. Time has been a major subject of religion, philosophy, and science, but defining it in a manner applicable to all fields of study without circularity has consistently eluded scholars. A simple definition states that "time is what clocks measure".

Time is one of the seven fundamental physical quantities in the International System of Units. Time is used to define other quantities — such as velocity — so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.