In the Essay Concerning Human Understanding, Locke explains how we come out with ideas of duration, eternity and time.  

We receive ideas of duration and measures of it from reflection and sensation, two foundations of knowledge. First, we observe how our ideas are formed in our minds: some begin to appear, others vanish. So, there is a constant succession of one idea by another. Observing a distance between the appearance of any two ideas, we get the idea of duration. The constant train of ideas in our minds furnishes us with the idea of existence, as we know that we do exist by this succession of ideas. The next thing is to measure the periods between ideas. This is how we get the ideas of minutes, hours, days, years, etc. Repeating these ideas of stated length of duration in our minds, we can frame the ideas of something that does not really exist. For instance, we can image tomorrow, next year. We can always add one idea of duration to another and apply them without bounds or limits. If we apply, for example, the length of the annual motion of the sun to durations past or to come without ever coming to the end of such addition, we come by the idea of eternity. By the same means, we can get the idea of eternal soul or of the infinite Being which must necessarily have always existed. Finally, when we consider this infinite duration by certain periods, we get the idea of what we call time in general.

Bibliography:

  • Locke, John, Essay Concerning Human Understanding in Modern Philosophy, an anthology of primary sources, second edition, edited by Roger Ariew and Eric Watkins, Hackett Publishing Company, Inc. 2009