The time separation at feeder fixes is initially strategic... the gap to achieve the landing rate becomes the tactical element (although most of the time they are the same)... If a pilot turns up 30 seconds early then so be it, hope the one ahead isn't 30 seconds late... This is when tactical control will be applied, vectors, speed or both, probably resulting in the crew blaming ATC for not knowing what they are doing...
In the ATC world 30 seconds is effectively the exact time. ie 20 and 30 seconds is the same minute as 21 and 29 seconds... both are on time for time 21...