Apple documentation lacks milliseconds. This example illustrates use.

dispatch_time_example.m

#import <Foundation/Foundation.h>

int main(int argc, char *argv[]) 
{
	@autoreleasepool {
		// Arg 2 is nanoseconds.
		//
		// https://developer.apple.com/documentation/dispatch/1420519-dispatch_time
		dispatch_time_t now = dispatch_time(DISPATCH_TIME_NOW, 0);
		
		// Add 1 second for reference.
		dispatch_time_t second = dispatch_time(now, 1 * NSEC_PER_SEC);
		
		// NSEC_PER_MSEC has, "No overview available," because Apple sucks. So, I 
		// assume it means milliseconds. If I times it by 100, I get tenths of a 
		// second. 
		//
		// https://developer.apple.com/documentation/dispatch/nsec_per_msec?language=objc
		dispatch_time_t tenth = dispatch_time(now, 1 * NSEC_PER_MSEC * 100);
		
		// Then I compare the three.
		NSLog(@"%llu", now);
		NSLog(@"%llu", tenth);
		NSLog(@"%llu", second);
	}
	
	return 0;
}