Now a pair of experimental atomic clocks have set a new record for timekeeping.
Every one of their ticks matches every other with a precision that means they vary in length by less than two parts of a quintillion - one followed by 18 zeroes.
The clocks are based on thousands of ytterbium atoms cooled to 10 millionths of a degree above absolute zero - the coldest temperature possible - and trapped in a laser lattice. A transition between two energy levels in the atoms acts as the clock's pendulum. The more stable a clock's ticks are, the more precise it can be.
Scientists at the National Institute of Standards and Technology (Nist) in the US tested the stability of the two ytterbium clocks by comparing their performance.
The findings, in the journal Science, may pave the way for improvements to devices that rely on accurate timekeeping such as GPS satellites. Nist physicist Dr Andrew Ludlow said: "The stability of the ytterbium lattice clocks opens the door to a number of exciting practical applications of high-performance timekeeping."