Currently, time is measured on the idea of a pendulum, or in the case of an atomic clock, the natural back-and-forth rhythm of a caesium atom. Similarly, the length of a second is defined as the time that passes during 9,192,631,770 cycles of the microwave signal generated by this process.
There is however another, more accurate, way of keeping time.
The optical clock, which unlike regular atomic clocks works using strontium atoms in place of caesium, operates at the visible, as oppose to the microwave, part of the electromagnetic spectrum.
Optical clocks offer a much higher level of accuracy than any other type of clock to the extent that, if had you started one running at the beginning of the universe, it would have only lost two minutes.
Read More: Unexplained Mysteries