The GPS clock system is based on atomic clock technology. Each of the 31 working satellites in the GPS network has a highly-accurate atomic clock built-in. These atomic clocks are synchronized and guided by a single master clock on the ground which is controlled by the United States Naval Observatory. 

MALTIME GPS clock systems have a time transfer accuracy of 40 nanoseconds (0.000 000 040 Second!) or less, meaning it takes almost no time at all for them to receive the correct time from GPS satellites and correct their own timekeeping accordingly.