Time is simply change. If nothing changed, there would be no time.
Past, present, and future are really just abstract ideas to make it more convenient to talk about change; they don’t actually exist. All that exists are systems and change, and the models that represent those systems and changes.
The past refers to changes that have already occurred. The future refers to changes that have yet to occur. Thus both are abstractions; neither actually exists. Strictly speaking, there is no “present” moment, since change requires movement through space, and the space over which the movement takes place can be infinitely reduced. It’s useful to think of the present as merely the moment when potential change becomes actual change.
What we call the future is actually just a state of a model of a system projected forward in time. We can imagine it in our minds, but it does not really exist in the external world. Even the phrase “projected forward in time” is just a convenient way of saying “the causes and effects defined in the model are imagined to have occurred, one after the other, to change the state of the model.”
This also explains why every device that supposedly measures time is actually measuring change. Or rather tiny changes, that occur one after the other.
From Wikipedia on 10-Nov-2012
Time is a dimension in which events can be ordered from the past through the present into the future and also the measure of durations of events and the intervals between them. Time has long been a major subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars. Nevertheless, diverse fields such as business, industry, sports, the sciences, music, dance, and the live theater all incorporate some notion of time into their respective measuring systems. Some simple, relatively uncontroversial definitions of time include “time is what clocks measure” and “time is what keeps everything from happening at once”.
Two contrasting viewpoints on time divide many prominent philosophers. One view is that time is part of the fundamental structure of the universe—a dimension independent of events, in which events occur in sequence. Sir Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time. The opposing view is that time does not refer to any kind of “container” that events and objects “move through”, nor to any entity that “flows”, but that it is instead part of a fundamental intellectual structure (together with space and number) within which humans sequence and compare events. This second view, in the tradition of Gottfried Leibniz and Immanuel Kant, holds that time is neither an event nor a thing, and thus is not itself measurable nor can it be travelled.
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.
Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined in terms of radiation emitted by caesium atoms (see below). Time is also of significant social importance, having economic value (“time is money”) as well as personal value, due to an awareness of the limited time in each day and in human life spans.