Time should be defined as the dimension of change because it passes. In this figurative talk, I believe, there is some truth that everyone should admit. In this paper, I aim to show what is behind the idea of time's passage, based on the presentist program originated by Arthur Prior, the founder of tense logic. For Prior, the presently existing things, and how they are, exhaust reality. Assuming this, he maintains that some statements may change in their truth-values since things, which those statements are about, do change in their properties. This is just what we mean by saying that time passes. I shall clarify and defend Prior's line of thought in the contemporary context of philosophy of time.