Don't use [NSDate timeIntervalFromReferenceDate] if you're working with deltas or timing or anything that isn't directly date related. Use absolute, monotonically increasing time instead. Using mach time is easy but slightly inconvenient; here's a very simple NSDate-like class you can use in your code.

GFClock.h

#import <Foundation/Foundation.h>

@interface GFClock : NSObject
+ (instancetype)sharedClock;
// since device boot or something. Monotonically increasing, unaffected by date and time settings
- (NSTimeInterval)absoluteTime;
@end

GFClock.m

#import "GFClock.h"
#include <mach/mach.h>
#include <mach/mach_time.h>

@implementation GFClock
{
	mach_timebase_info_data_t _clock_timebase;
}

+ (instancetype)sharedClock
{
	static GFClock *g;
	static dispatch_once_t onceToken;
	dispatch_once(&onceToken, ^{
		g = [GFClock new];
	});
	return g;
}

- (id)init
{
	if(!(self = [super init]))
		return nil;
	mach_timebase_info(&_clock_timebase);
	return self;
}

- (NSTimeInterval)absoluteTime
{
	uint64_t machtime = mach_absolute_time();
	uint64_t nanos = (machtime * _clock_timebase.numer) / _clock_timebase.denom;
	
	return nanos/1.0e9;
}
@end

How about CACurrentMediaTime()?

Could use NSEC_PER_SEC instead of 1.0e9.

@intelliot seems odd to link against Core Animation for something that doesn't use animation?