Your phone's GPS would drift 10 km a day without correcting for relativity

GPS satellites move fast enough that special relativity slows their onboard clocks by about 7 microseconds a day, but they also sit in weaker gravity, which general relativity speeds those same clocks up by about 45 microseconds a day. Engineers pre-tune every satellite clock to cancel the net 38-microsecond drift — skip it, and positions would be wrong by kilometres within a day.

— Neil Ashby, Relativity in the Global Positioning System — Living Reviews in Relativity, 2003