What time a clock reads can be set by any timekeeper, but physics determines how quickly time passes. In the early years of the 20th century, Albert Einstein determined that two observers won't agree on how long an hour is if they aren't moving at the same speed in the same direction. That disagreement also holds between a person on Earth's surface and another in orbit or on the Moon.

.....the Moon's motion relative to ours makes clocks run slower than Earth standard, but its lower gravity leads to clocks running faster. "So these are two competing effects, and the net result of this is a 56-microseconds-per-day drift." (That's 0.000056 second.)

Though a 56-microsecond difference is small by human standards, it's significant when it comes to guiding multiple missions with pinpoint accuracy or communicating between Earth and the Moon.