A popular metaphor for time is that the future lies ahead of us, the present is beside us, and the past is behind us. One variation is that we are moving into the future. Another variation is that we are standing still, as observers, while time is moving past us. This is called the passage of time. But there are philosophers and physicists who deny the passage of time.
There are roughly three views of time. On one view, only the present is real. The past is no longer real, although it's unalterable, and the future will become real. Another position is the growing block view of time, where past and present are real, but the future is unreal. Then there's the B-theory, according to which the entire timeline is real.
Why does time seem to be pass? I'm going to offer one suggestion. I don't say this is true. But it's consistent with experience. If successful, it has empirical adequacy, which may fall short of proving it true.
Time seems to flow or move because our consciousness only samples increments of time. Moreover, it samples increments of time in a realistic sequence. I'm directly aware of one moment, then the next moment, then the next moment. I remember earlier moments, so I can mentally compare them, but I consciously experience one moment at a time, in a coherent series.
It's like watching a movie. You can only see one frame at a time, but each frame is sequential with what precedes it or succeeds it, not in the sense of an arbitrary series of disparate incidents, but where the next moment seems to grow out of the previous moment, in a cause-effect relation. So there's apparent continuity. What happens a moment later is related to what happened a moment before. You seen the plot unfold, one incident at a time.
On this view, it's possible that the observer actually exists all at once, at different times, but he doesn't perceive reality all at once. Like a radio in which many stations coexist, but a listener can only tune into one channel at a time.
Let's say it's 2017, and our observer was born in 2000. Let's say that he's immortal in the Christian sense that even though he will die, yet having once come into existence, he will never pass out of existence. Suppose, therefore, his existence is an actual infinite, beginning in 2000, and extending into future eternity. So he actually exists in the future, and he actually exists in the past, as well as the present. There's some asymmetry to this comparison, because his future existence has no cutoff, whereas his past existence only extends back to 2000. He didn't exist before then.
On this view, he can only experience sequential segments of time even though he actually exists before and after what he is able to experience. Although he exists in the future, as well as the present and the past, he isn't conscious of the future. His potential awareness of the future is blocked, like there's a filter that screens out the past and the future, so that he can only perceive the shifting present, in incremental intervals.
For this to work, his perception of time must be synchronized with other humans. Everyone who's alive in 2017 has an awareness of the same moment in 2017. Coordinated perceptions of the same moment, and the next moment, and the next moment. They can perceive a particular moment in 2017 because they are in 2017, but they are also in 2016, and 2018, and 3000. Even though they are actually in all these different times, they can't perceive them all at once.
It's analogous to science fiction stories where test-subjects lying on rows of hospital beds, in induced comas, with a neurointerface feeding simulated stimuli into their brains. They all find themselves in the same virtual world, in the same timeframe, with the same setting. A kind of preestablished harmony.