How to log a method's execution time exactly in milliseconds?

NSDate *methodStart = [NSDate date];

/* ... Do whatever you need to do ... */

NSDate *methodFinish = [NSDate date];
NSTimeInterval executionTime = [methodFinish timeIntervalSinceDate:methodStart];
NSLog(@"executionTime = %f", executionTime);

Swift:

let methodStart = NSDate()

/* ... Do whatever you need to do ... */

let methodFinish = NSDate()
let executionTime = methodFinish.timeIntervalSinceDate(methodStart)
print("Execution time: \(executionTime)")

Swift3:

let methodStart = Date()

/* ... Do whatever you need to do ... */

let methodFinish = Date()
let executionTime = methodFinish.timeIntervalSince(methodStart)
print("Execution time: \(executionTime)")

Easy to use and has sub-millisecond precision.

Here are two one-line macros that I use:

#define TICK   NSDate *startTime = [NSDate date]
#define TOCK   NSLog(@"Time: %f", -[startTime timeIntervalSinceNow])

Use it like this:

TICK;

/* ... Do Some Work Here ... */

TOCK;

For fine-grained timing on OS X, you should use mach_absolute_time( ) declared in <mach/mach_time.h>:

#include <mach/mach_time.h>
#include <stdint.h>

// Do some stuff to setup for timing
const uint64_t startTime = mach_absolute_time();
// Do some stuff that you want to time
const uint64_t endTime = mach_absolute_time();

// Time elapsed in Mach time units.
const uint64_t elapsedMTU = endTime - startTime;

// Get information for converting from MTU to nanoseconds
mach_timebase_info_data_t info;
if (mach_timebase_info(&info))
   handleErrorConditionIfYoureBeingCareful();

// Get elapsed time in nanoseconds:
const double elapsedNS = (double)elapsedMTU * (double)info.numer / (double)info.denom;

Of course the usual caveats about fine-grained measurements apply; you're probably best off invoking the routine under test many times, and averaging/taking a minimum/some other form of processing.

Additionally, please note that you may find it more useful to profile your application running using a tool like Shark. This won't give you exact timing information, but it will tell you what percentage of the application's time is being spent where, which is often more useful (but not always).