If it's mission critical then -- in Europe -- it gets the time from DCF77. It uses an atomic clock to emit a time signal (actually, it uses three and only uses

If it's mission critical then -- in Europe -- it gets the time from DCF77. It uses an atomic clock to emit a time signal (actually, it uses three and only uses their output when two out of three agrees). It is also compared to further atomic clocks and corrected as necessary. Even many consumer grade clocks will sync to the radio signal once a day and use a quartz otherwise.