Getting time elapsed in Objective-C

Solution 1:

NSDate *start = [NSDate date];
// do stuff...
NSTimeInterval timeInterval = [start timeIntervalSinceNow];

timeInterval is the difference between start and now, in seconds, with sub-millisecond precision.

Solution 2:

You should not rely on [NSDate date] for timing purposes since it can over- or under-report the elapsed time. There are even cases where your computer will seemingly time-travel since the elapsed time will be negative! (E.g. if the clock moved backwards during timing.)

According to Aria Haghighi in the "Advanced iOS Gesture Recognition" lecture of the Winter 2013 Stanford iOS course (34:00), you should use CACurrentMediaTime() if you need an accurate time interval.

Objective-C:

#import <QuartzCore/QuartzCore.h>

CFTimeInterval startTime = CACurrentMediaTime();
// perform some action
CFTimeInterval elapsedTime = CACurrentMediaTime() - startTime;

Swift:

let startTime = CACurrentMediaTime()
// perform some action
let elapsedTime = CACurrentMediaTime() - startTime

The reason is that [NSDate date] syncs on the server, so it could lead to "time-sync hiccups" which can lead to very difficult-to-track bugs. CACurrentMediaTime(), on the other hand, is a device time that doesn't change with these network syncs.

You will need to add the QuartzCore framework to your target's settings.

Note: There are other API's like clock_gettime_nsec_np that may work as well.