Modifications to the world's most accurate clock means it is not expected to lose or gain a second in 15 billion years - a period of time greater than the age of the universe.
The experimental strontium lattice clock built by scientists in the United States is now more than three times as precise as it was last year when it set the previous world record.
The clock's stability - how closely each tick matches every other tick - has also been improved by almost 50%.
It is now sensitive enough to measure tiny changes in the passage of time at slightly different heights, an effect predicted by Albert Einstein in his theory of general relativity.
Because of the way gravity warps space-time, clocks should run slower the closer they are to a massive object, according to the theory.
Dr Jun Ye, from the JILA institute which developed the clock, said: "Our performance means that we can measure the gravitational shift when you raise the clock just two centimetres on the Earth's surface."
In the JILA clock, a few thousand grams of strontium are held by intense laser light in a column called an optical lattice.
The 430 trillion "ticks" per second of strontium atoms oscillating between two energy levels are detected by bathing the atoms in very stable red laser light at a precise frequency.
The improvements involved reducing clock errors related to heat from the surrounding environment, called blackbody radiation. This produces an electric field that alters the atoms' response to laser light.
JILA is a joint institute of the National Institute of Standards and Technology (Nist) and the University of Colorado at Boulder.
A description of the clock's new performance appears in the journal Nature Communications.