Special theory of relativity

In the special theory of relativity, Einstein, in his first postulated, redeveloped the simultaneity saying that the speed of light is constant and independent of any sistem of reference. Also he created another postulated in which, he said that if a flash of light is thrown seen it by two observers, in relative motion, both see the light away producing a perfect circle with each of them in the center. If in both sides, we put a detector, none of the observers would agree on what detector was activated first. 
Because of that, we can confirm that now, with Einstein, the concepts of absolute time and simultaneity are lost.