Convert dispatch_time_t to NSNumber (ms)

ConvertDispatchToMs.m

/**
 * Convert a dispatch_time_t to milliseconds.
 * note that this only works if dispatch time is in the future,
 * and is approximate (since the time now will be slightly different than
 * the time you made the dispatch_time_t).
 */
static NSNumber * dispatch_future_to_ms(dispatch_time_t time) {
  uint64_t abs = mach_absolute_time();
  return @((time - abs) / NSEC_PER_MSEC);
}