What is the meaning of the concept of time at the beginning of the "Big Bang" before cesium existed to measure the length of a second and while the Universe was expanding faster than the speed of light?

Currently we measure the length of a second as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom at rest at a temperature of 0 Kelvin. Since cesium did not exist in the early Universe, and the Universe was supposedly undergoing a bizarre expansion how is it meaningful to apply the concept of time to that early Universe?