What you learned in school
Physics textbooks taught Newton's concept of absolute time - that time flows uniformly throughout the universe like a cosmic clock. Students learned that one second on Earth equals one second everywhere else, and that time was an unchanging universal constant. This absolute time was fundamental to all classical physics calculations and seemed obviously true.
What we know now
Physics textbooks taught that time flows at the same rate everywhere in the universe - a universal constant that's the same for all observers. Newton's absolute time was fundamental to classical physics education. Students learned that time was like a cosmic clock ticking uniformly. Einstein's special and general relativity theories showed that time is relative and can dilate (slow down) depending on velocity and gravitational fields. Time passes differently for observers moving at different speeds or in different gravitational environments. GPS satellites must account for these effects to maintain accuracy.