That simple question probably is asked more often today than ever. In our clock-studded, cell-phone society, the answer is never more than a glance away, and so we can blissfully partition our days into ever smaller increments for ever more tightly scheduled tasks, confident that we will always know it is 7:03 P.M.

Modern scientific revelations about time, however, make the question endlessly frustrating. If we seek a precise knowledge of the time, the elusive infinitesimal of “now” dissolves into a scattering flock of nanoseconds. Bound by the speed of light and the velocity of nerve impulses, our perceptions of the present sketch the world as it was an instant ago—for all that our consciousness pretends otherwise, we can never catch up.

Even in principle, perfect synchronicity escapes us. Relativity dictates that, like a strange syrup, time flows slower on moving trains than in the stations and faster in the mountains than in the valleys. The time for our wristwatch or digital screen is not exactly the same as the time for our head. It is roughly 7:04 P.M.

Our intuitions are deeply paradoxical. Time heals all wounds, but it is also the great destroyer. Time is relative but also relentless. There is time for every purpose under heaven, but there is never enough. Time flies, crawls and races. Seconds can be both split and stretched. Like the tide, time waits for no man, but in dramatic moments it also stands still. It is as personal as the pace of one’s heartbeat but as public as the clock tower in the town square. We do our best to reconcile the contradictions. It seems like 7:05 P.M.

And of course, time is money. It is the partner of change, the antagonist of speed, the currency in which we pay attention. It is our most precious, irreplaceable commodity. Yet still we say we don’t know where it goes, and we sleep away a third of it, and none of us really can account for how much we have left. We can find 100 ways to save time, but the amount remaining nonetheless diminishes steadily. It is already 7:06 P.M.

Time and memory shape our perceptions of our own identity. We may feel ourselves to be at history’s mercy, but we also see ourselves as free-willed agents of the future. That conception is disturbingly at odds with the ideas of physicists and philosophers, however, because if time is a dimension like those of space, then yesterday, today and tomorrow are all equally concrete and determined. The future exists as much as the past does; it is just in a place that we have not yet visited. Somewhere it is 7:07 P.M.

“Time is the substance of which I am made,” Argentine writer Jorge Luis Borges wrote. “Time is a river that carries me away, but I am the river; it is a tiger that destroys me, but I am the tiger; it is a fire that consumes me, but I am the fire.” This special issue of Scientific American summarizes what science has discovered about how time permeates and guides both our physical world and our inner selves. That knowledge should enrich the imagination and provide practical advantages to anyone hoping to beat the clock or at least to stay in step with it. It is now 7:08 P.M. Synchronize your watches.