The world's most accurate clocks lose one second every 300 million years

Optical atomic clocks, developed at institutions including NIST (USA) and PTB (Germany), are now so precise that they would lose or gain only about one second over 300 million years. They measure the oscillation of atoms (typically aluminium or ytterbium ions) at frequencies around 1 quadrillion cycles per second. These clocks are already so accurate that they can detect the gravitational time dilation predicted by Einstein's general relativity over a height difference of just a few centimetres: a clock at waist height ticks measurably slower than one at eye level. GPS and financial trading infrastructure both depend on atomic clock precision.

Source: McGrew, W.F. et al., 'Atomic clock performance', Nature (2018)