#import <mach/mach_time.h>

uint64_t start = mach_absolute_time();

// do stuff to be timed

uint64_t end = mach_absolute_time();
uint64_t elapsed = end - start;

mach_timebase_info_data_t info;
if (mach_timebase_info (&info) != KERN_SUCCESS) {
    printf ("mach_timebase_info failed\n");
}

uint64_t nanosecs = elapsed * info.numer / info.denom;
uint64_t millisecs = nanosecs / 1000000;