I have also tried to get the borders this way (cat keeps locked showing event by event, head shows only the first n):

system("head -n 1 /dev/gpio-event");
gettimeofday(&start, NULL);

system("head -n 1 /dev/gpio-event");
gettimeofday(&stop, NULL);

Despite I've set EventBothEdges and the frequency is very low it quickly start to lose events, and it only shows rising edges (when set to show only falling it does, but still losing events).

114 R 1289454882.723571
GPIO Event: event lost due to queue full
114 R 1289454882.739990
GPIO Event: event lost due to queue full
114 R 1289454882.924774
GPIO Event: event lost due to queue full
114 R 1289454882.950866
GPIO Event: event lost due to queue full
114 R 1289454883.132141
GPIO Event: event lost due to queue full
114 R 1289454883.15