Because Earth is not the best time keeper we now measure a second by the number of flicks of frequency wavelength of radiation emitted by atoms of cacsium (9,192,631,770 flicks a second ) .
But whats gets me is this " if you could measure a second near a black hole you would get the same number of flicks per second but those not near the black hole would find more time had passed , being near the black hole event horizon does effect the light of the oscillations of electromagnetic waves including the atoms we are made of .
As Einstein realized , light provides the only fundamental measure of both length & time in the universe .
If you have read this far I'm astounded
Cheers Bobby.