How can I get a precise time, for example in milliseconds in Objective-C?

NSDate and the timeIntervalSince* methods will return a NSTimeInterval which is a double with sub-millisecond accuracy. NSTimeInterval is in seconds, but it uses the double to give you greater precision.

In order to calculate millisecond time accuracy, you can do:

// Get a current time for where you want to start measuring from
NSDate *date = [NSDate date];

// do work...

// Find elapsed time and convert to milliseconds
// Use (-) modifier to conversion since receiver is earlier than now
double timePassed_ms = [date timeIntervalSinceNow] * -1000.0;

Documentation on timeIntervalSinceNow.

There are many other ways to calculate this interval using NSDate, and I would recommend looking at the class documentation for NSDate which is found in NSDate Class Reference.

mach_absolute_time() can be used to get precise measurements.

See http://developer.apple.com/qa/qa2004/qa1398.html

Also available is CACurrentMediaTime(), which is essentially the same thing but with an easier-to-use interface.

(Note: This answer was written in 2009. See Pavel Alexeev's answer for the simpler POSIX clock_gettime() interfaces available in newer versions of macOS and iOS.)

CFAbsoluteTimeGetCurrent() returns the absolute time as a double value, but I don't know what its precision is -- it might only update every dozen milliseconds, or it might update every microsecond, I don't know.

I would NOT use mach_absolute_time() because it queries a combination of the kernel and the processor for an absolute time using ticks (probably an uptime).

What I would use:

CFAbsoluteTimeGetCurrent();

This function is optimized to correct the difference in the iOS and OSX software and hardware.

Something Geekier

The quotient of a difference in mach_absolute_time() and AFAbsoluteTimeGetCurrent() is always around 24000011.154871

Here is a log of my app:

Please note that final result time is a difference in CFAbsoluteTimeGetCurrent()'s

 2012-03-19 21:46:35.609 Rest Counter[3776:707] First Time: 353900795.609040
 2012-03-19 21:46:36.360 Rest Counter[3776:707] Second Time: 353900796.360177
 2012-03-19 21:46:36.361 Rest Counter[3776:707] Final Result Time (difference): 0.751137
 2012-03-19 21:46:36.363 Rest Counter[3776:707] Mach absolute time: 18027372
 2012-03-19 21:46:36.365 Rest Counter[3776:707] Mach absolute time/final time: 24000113.153295
 2012-03-19 21:46:36.367 Rest Counter[3776:707] -----------------------------------------------------
 2012-03-19 21:46:43.074 Rest Counter[3776:707] First Time: 353900803.074637
 2012-03-19 21:46:43.170 Rest Counter[3776:707] Second Time: 353900803.170256
 2012-03-19 21:46:43.172 Rest Counter[3776:707] Final Result Time (difference): 0.095619
 2012-03-19 21:46:43.173 Rest Counter[3776:707] Mach absolute time: 2294833
 2012-03-19 21:46:43.175 Rest Counter[3776:707] Mach absolute time/final time: 23999753.727777
 2012-03-19 21:46:43.177 Rest Counter[3776:707] -----------------------------------------------------
 2012-03-19 21:46:46.499 Rest Counter[3776:707] First Time: 353900806.499199
 2012-03-19 21:46:55.017 Rest Counter[3776:707] Second Time: 353900815.016985
 2012-03-19 21:46:55.018 Rest Counter[3776:707] Final Result Time (difference): 8.517786
 2012-03-19 21:46:55.020 Rest Counter[3776:707] Mach absolute time: 204426836
 2012-03-19 21:46:55.022 Rest Counter[3776:707] Mach absolute time/final time: 23999996.639500
 2012-03-19 21:46:55.024 Rest Counter[3776:707] -----------------------------------------------------

Functions based on mach_absolute_time are good for short measurements.
But for long measurements important caveat is that they stop ticking while device is asleep.

There is a function to get time since boot. It doesn't stop while sleeping. Also, gettimeofday is not monotonic, but in my experiments I've always see that boot time changes when system time changes, so I think it should work fine.

func timeSinceBoot() -> TimeInterval
{
    var bootTime = timeval()
    var currentTime = timeval()
    var timeZone = timezone()

    let mib = UnsafeMutablePointer<Int32>.allocate(capacity: 2)
    mib[0] = CTL_KERN
    mib[1] = KERN_BOOTTIME
    var size = MemoryLayout.size(ofValue: bootTime)

    var timeSinceBoot = 0.0

    gettimeofday(&currentTime, &timeZone)

    if sysctl(mib, 2, &bootTime, &size, nil, 0) != -1 && bootTime.tv_sec != 0 {
        timeSinceBoot = Double(currentTime.tv_sec - bootTime.tv_sec)
        timeSinceBoot += Double(currentTime.tv_usec - bootTime.tv_usec) / 1000000.0
    }
    return timeSinceBoot
}

And since iOS 10 and macOS 10.12 we can use CLOCK_MONOTONIC:

if #available(OSX 10.12, *) {
    var uptime = timespec()
    if clock_gettime(CLOCK_MONOTONIC_RAW, &uptime) == 0 {
        return Double(uptime.tv_sec) + Double(uptime.tv_nsec) / 1000000000.0
    }
}

To sum it up:

• Date.timeIntervalSinceReferenceDate — changes when system time changes, not monotonic
• CFAbsoluteTimeGetCurrent() — not monotonic, may go backward
• CACurrentMediaTime() — stops ticking when device is asleep
• timeSinceBoot() — doesn't sleep, but might be not monotonic
• CLOCK_MONOTONIC — doesn't sleep, monotonic, supported since iOS 10