That most alarming New Year’s morning question — “Uh-oh, what did I do last night?” — can seem benign compared with those that may come later, like “Uh, what exactly did I do with the last year?”

Or, “Hold on — did a decade just go by?”

It did. Somewhere between trigonometry and colonoscopy, someone must have hit the fast-forward button. Time may march, or ebb, or sift, or creep, but in early January it feels as if it has bolted like an angry dinner guest, leaving conversations unfinished, relationships still stuck, bad habits unbroken, goals unachieved.

“I think for many people, we think about our goals, and if nothing much has happened with those then suddenly it seems like it was just yesterday that we set them,” said Gal Zauberman, an associate professor of marketing at the Wharton School of Business.

Yet the sensation of passing time can be very different, Dr. Zauberman said, “depending on what you think about, and how.”

In fact, scientists are not sure how the brain tracks time. One theory holds that it has a cluster of cells specialized to count off intervals of time; another that a wide array of neural processes act as an internal clock.

Either way, studies find, this biological pacemaker has a poor grasp of longer intervals. Time does seem to slow to a trickle during an empty afternoon and race when the brain is engrossed in challenging work. Stimulants, including caffeine, tend to make people feel as if time is passing faster; complex jobs, like doing taxes, can seem to drag on longer than they actually do.

And emotional events — a breakup, a promotion, a transformative trip abroad — tend to be perceived as more recent than they actually are, by months or even years.