America’s official time is kept at a government laboratory in Boulder, Colo., and according to the clock at the entrance, I was seven minutes behind schedule. I rush across the campus of the National Institute of Standards and Technology (NIST) and arrive at the end of a long hallway where physicist Jeff Sherman was waiting patiently.

“Sorry I’m running late,” I tell him.

“It’s OK, we only measure the nanoseconds,” Sherman jokes.

It’s never been easier to know what time it is. NIST broadcasts the time to points across the country. It’s fed through computer networks and cellphone towers to our personal gadgets, which tick in perfect synchrony. Humanity’s ever-improving agreement on the time smooths communication and transportation, and it lubricates our economy. But time has another side to it, one that the clocks don’t show. “A lot of us grow up being fed this idea of time as absolute,” says Chanda Prescod-Weinstein, a theoretical physicist at the University of New Hampshire. But Prescod-Weinstein says the time we’re experiencing is a social construct. Real time is actually something quite different. In some of the odder corners of the Universe, space and time can stretch and slow — and sometimes even break down completely.